Tag Archive: acrylic

Jeff Enswieler 4

We are occasionally asked what has caused a localized discoloration in a spa.  Most of the time, this change in color appears as a bleached surface.  There have even been examples where tiny blisters were present in the bleached area   In every case we have investigated, the root cause has been localized exposure to a strong sanitizing chemical.

The majority of spas are produced from  a  cross-linked, high molecular weight acrylic polymer sheet, a material that is very resistant to chemicals.  However, no product is perfect when it involves chemical resistance.   Spas require the use of sanitizing chemicals to maintain water chemistry.   Manufacturers provide recommendations on what products to use, and how to use them.  Failure to comply with these recommendations can lead to problems.  Even if a product might be used in a swimming pool, it may have issues when used in a spa.

Pure acrylic is clear with transparency better than most glass.  Pigments are added to the liquid acrylic to create the attractive colors and effects.  New colors often use exotic pigments.  Sanitizers for water are oxidizing agents used to kill bacteria, algae, etc.  Some are stronger than others.  The most common spa sanitizers are compounds of chlorine or bromine.   Some of these products have been found to bleach the acrylic pigments, and cause discoloration.  Manufacturers recommendations are always designed to prevent concentrated chemicals from long contact with the acrylic surface, and the plumbing system.  Water treatment chemicals for spas are best when in liquid form since they are easier to control concentration.  All examples of spa surface discoloration, that have been reported to MTP, have been caused by  one of these water treatment products.

Inorganic chlorine compounds such as calcium hypochlorite and sodium hypochlorite  commonly used sanitizers for swimming pools and spas.  However, they are both effected by sunlight, so they require the addition of cyanuric acid to stabilize the chlorine in outdoor pools and spas.  Organic chlorine compounds are products combined with cyanuric acid. Sodium dichlorostriazinetrione (“dichlor’) and trichloro-s-triazinetri-one (‘trichlor) are both popular stabilized chlorines for water treatment of outdoor pools.  Most of the spa discoloration cases we have seen have been the result of the use of “tri-chlor”.  Specifically, they have been due to the use of this chemical in the form of solid tablet.  This product dissolves slowly in water, and is a very strong oxidizer as well as increases the acidity of the water.  When these tablets are in contact or close proximity to the acrylic surface, it will attack the acrylic pigments and cause bleaching, and sometimes blistering.  Di-Chlor is a weaker chemical, and should be the stabilizer of choice for spas and is available in granular form.  Furthermore, when any sanitizer or acid is added to spa water, the circulating pump and jets should be running to assure rapid mixing of the chemical into the water.  This avoids creating localized regions of highly concentrated chemicals.

These problems have been verified by one of the acrylic sheet manufacturers.  They performed extensive testing of several water treatment products, and found that ‘trichlor” tablets can cause the bleaching similar to the examples that we have  observed and have been reported to us. They also found that when combined with hot water, it would cause blistering.  Some cases reported to MTP showed a red, rust-like material in the bleached area.  Their tests duplicated this phenomena when there was a source of iron in the water.   High concentration of the oxidizing chemical is a prerequisite for the discoloration.  This is facilitated by tablet form chemicals, since they can lay on the acrylic surface while dissolving.  If water is not being circulated, there will be a very high concentration at the interface of the acrylic and the tablet.

chem bleach1chem bleach

The two photos above show discoloration and blistering from the incorrect use of chemicals like “tri-chlor”.

Some people like to use floaters with a chemical inside.  Although it is a convenient way to treat water, it is not recommended, since it can cause problems.  The floater can rest against the side of the spa, and since it is in close proximity, it can cause bleaching of the surface.

Poor chemical control can also cause problems within the plumbing system by reacting with sealants, o-rings, etc.  There have  been reports of other spa components being compromised the use of bromine sanitizers, and ozonators (direct ozone addition).   Any chemical should be quickly dissolved and diluted with water to avoid problems.   It is very important for a spa owner to maintain total water chemistry (e.g. pH, chlorine, total solids, etc) as recommended by the manufacturer.  Various problems can occur if chemistry gets out of control.  For example, if chlorine level is too high the vapors  can bleach the under surface of a spa cover.   Excessive chemicals in water can also damage the jets, controls and fixtures.  Check with your manufacturer for their recommendations on how to maintain water chemistry in your spa.

Sometimes, the bleached surface can be removed by sanding and re-polishing, so surface appearance can be recovered.  However,  avoid repeating the factors which caused the original discoloration.

Following are some photos that provide examples to chemical bleaching (chemical burns).  The owner of at least one of these samples reported seeing pits in the bleached area.   There are chemicals that have been known to attack cross-linked acrylic.  One example is Wintergreen oil.  Although, the normal mode of failure from chemical attack on an acrylic surface is stress cracking, failure can occur in various ways.  Failure in a spa caused by wintergreen oil have been seen where it looked like a liquid had been poured on the surface , and it ran down the side.  The acrylic was discolored, and microscopic crazing was present.  Refer to information on stress-cracking in the website, http://www.multitechproducts.com.   So, unless a chemical has been approved for use in a spa, avoid using it.

The pictures above illustrate these problems in solid color and marble (multiple colored patterns) color spas.

The apparent defect can also occur in “granite” spas as shown below.    Again, they can be repaired using a sanding procedure.  You would start by using 80 or 100 grit sandpaper until the discolored area is removed, and you see the original color.  Then you would finish by using 320 grit wet/dry paper, and finally 400 grit.  If granite texture needs to be regained, you can use Multi-Tech Products K2000 clear topcoat with a brush.  Use the procedure provided to add texture.

Granite discolor









The picture below gives an example of the worst case of chemical attack we have seen on an acrylic spa.  This severe, widespread discoloration would not lend itself to sanding, buffing, and polishing to recover the original color(s).  Read the message at the link for more information on cause and refinish options on this case.








Some general information on chemicals used for spas follows:

Calcium hypochlorite is available in granular or tablet form. It provides 65% available chlorine by weight and remains stable if stored in a dry, cool area. The chemical can be dissolved and introduced as a liquid, or it can be added in dry form. When
applied directly, it may cause a temporary cloudiness. Direct applications should be broadcast evenly over the water surface to avoid bleaching.
This chemical, when contaminated by or mixed with an organic compound, can produce a fire. A good rule is never to mix calcium hypochlorite with another chemical or store it in anything but the original container. Mix the chemical into water not water into the chemical. Calcium hypochlorite should not be handled with bare hands and must bekept off the operator’s clothes. As a chlorinating agent, calcium hypochlorite will slightly increase water pH. It’s pH is 11.8.

Sodium dichloro-s-triazinetrione and trichloro-s-tria zinetrione – chlorine compounds that contain cyanuric acid, and are used to stabilize chlorine.  The dichlor is more soluble and provides 56% or 62% available chlorine, depending on formulation.   The dichlor compound has little effect on pH, while trichlor is extremely acid (pH 2.8 -3.0).  Dichlor can be added directly to the water. The major effect of cyanuric acid on hypochlorous acid (HOCI) is to keep it from being decomposed by ultraviolet light contained in sunlight. Because it is readily decomposed by LTV light, the dosage of a chlorinating agent loke calcium hypochlorite that is sufficient for an indoor pool/spa is dissipated rapidly in an outdoor pool/spa.  Cyanuric acid bonds with the available chlorine in a manner that does not use up the chlorine.  At high stabilizer levels (over 100 ppm), chlorine’s efficiency may be reduced. The operator should consult the local codes and manufacturers’ recommendations on the proper use of stabilizers.


Ken Wolfe (Chemical Engineer & Consultant)


To:  Multi-Tech Products

I want to thank you for your assistance in supplying a great system and help to replace and renew part of the surface of a swim spa installed in a home in Alaska.  The spa was originally produced from a co-extruded polymeric sheet with fiberglass reinforced resin for mechanical support.  Due to the presence of un-cured resin in the support structure, blisters had occurred around the bottom of the spa.  They were too numerous and widespread to repair with your standard procedure.  Furthermore, as the picture below shows, it would have been very expensive to remove and replace the entire spa   You can also see a photo showing some of the blisters..


These blisters were located only around the lower portion of the spa, so your procedure for renewing a portion of a spa surface was ideal for this job.  As your procedures state, we created a line of demarcation establishing the boundary of the material to be removed and replaced.  Then we stripped this top layer from the spa with  the aid of a chisel and hammer.  We removed the entire bottom  of the spa.   Then we applied new fiberglass mat and resin using your FRL Kit to the affected surface.   See photos.

IMG_0684 IMG_0681IMG_0697

Padres Spa Pictures 223


We completed the job following your recommendations using the products in your kit.  The following photo shows the final result.  The home owner was ecstatic about the appearance, and our ability to repair it without the demolition of replacing the entire spa.


Thanks again,


To:  Jeff

I want to thank you for your testimonial on our FRL (Fiberglass Reinforced Lining) System.  It was designed especially for this application, and we have had many satisfied customers including the San Diego Padres baseball team where the spa in their locker room was renewed.  We also want to thank you for trusting and using all of our products to repair spas and bathtubs made from acrylics, gelcoat, and other materials.


Rob Clos


TO: Multi-Tech Products

I recently completed a cosmetic damage repair on a shower wall surround. I used my standard repair filler, color coating and methods. The repair failed as the filler fell out of the prepared area while the coating bonded well. I remember not recognizing the material but proceeded with my usual methods anyway. It was a small crack and the structure was solid. When I used my grinder to prepare the crack, I found a material with two or three layers. It had different colors to the layers. The thin bath ware top white color, a pure white plastic like layer, and the structural layer similar to a composite reinforced structure. The structure layer did not smell like fiberglass when I ground into it some. What do you think it was, and what is the proper way to repair it so it will hold?

Dear Steve;

It sounds like you found a product made from what is called “co-extruded” in the industry. This material is less expensive than traditional materials and is becoming more popular in economy products. You will find them at your local Home Centers. Manufacturers continually seek materials and processes to lower manufacturing costs. Historically, most bath tubs and shower products have been manufactured from cross-linked, cast acrylic sheet or gel coat. Both are reinforced with FRP (polyester resin with embedded glass fibers). Cast acrylic sheet is the most expensive acrylic on the market, and the cross-linked variety was specifically engineered for bathtubs and spas. Therefore, it has the best balance of physical and chemical properties for these applications.

The co-ex product you experienced is made by extruding a two-layer sheet – the top is a lower molecular weight, un-cross-linked acrylic, and the bottom is ABS (acrylonitrile-butadiene-styrene copolymer). Typically, the sheet is .125″ thick with the top layer being about .025″ and the bottom layer about .100″. When formed into bathtubs or shower walls, it can then be reinforced with FRP, polyurethane (PU), or have no reinforcement. PU may have glass fibers in the resin, and it can be a high density foam or a smooth coating. Sometimes it is reinforced with fiberboard strips or panels.

You mentioned there was an odor when grinding the structure. This is usually the best way to establish identity of a polyester resin. Simply use sand paper (320 grit) to sand the material in question before proceeding with a repair. Grinding may be required to expose the different layers. Polyester resins will have a distinct sweet smell with sanding or grinding friction. After the sanding test to confirm that it is a polyester, it is safe to use similar polyester resins for filling and reinforcement purposes. If there is no polyester odor, use our binding resin for reinforcement. It is formulated to adhere to PU and other plastic surfaces.

We would also recommend verifying the structural strength and integrity of the repair zone before applying the color coatings. Be certain there is no movement or stress to the area when hand pressure is applied. If there is, remove any filler and start over with the repair.

When repairing the cosmetic surface of bath products made with “co-ex” , our materials and processes work well. Our MMA or Quick Glaze systems perform very well as the color coating. In fact, since the lower molecular weight acrylic is not cross-linked, coatings adhere to it better. However, compared to polyester resins, it is more difficult to achieve adhesion on ABS or PU. So, you should not use Poly-Filler or any polyester resin based crack or putty fillers. The deceiving factor is that most polyester fillers will initially bond to the material for a short period.   However, after time it will lose its adhesion to the ABS or PU, and will break away from the repair zone. Our acrylic filler should be used for this application.

I hope this provides you the necessary information to perform the best repair.  If you need more help, please call or email.


Rob Clos

TO:  Multi-Tech Products

I am repair technician and I can never achieve a hidden repair with your MMA system on acrylic.
My filler and contour application is perfect, but I always have a dark ring effect around the repair area after buffing.
I know factories use this product to get perfect repairs. What do they do differently? One factory even sends me the color. So I know it is the right color.
What am I doing wrong?  See photo.


Repair Technician








Hello John,

This is one of the most common questions from repair technicians about achieving the perfect repair with our MMA System on acrylic bath and shower repairs.   The simple answer is the Clear Coat must be used.  Many technicians don’t understand the functions and importance of the clear coat, and therefore they think it is not necessary.

Some bath ware manufacturers send  only the product color coating (base coat) to technicians for warranty repairs. This certainly helps the contracted technician to use the proper color, but it might give the impression that only the base coat is needed for the repair.  I would also believe the customer service representative may not understand all of the requirements, as well.  However, the clear coat plays a role in both the dark ring, and the endurance of the repair.  The dark ring you mention develops when the base coat is sanded and polished without the presence of Clear Coat.

Since the MMA Basecoat is highly pigmented, it is not designed to withstand dirt, soap and general bath and shower environment usage.  Over a short period of time, the repair zone without Clear Coat will change color and become stained.  The Clear Coat is formulated to seal the base coat and give the repair its hardness, UV protection, and longevity.

How to remove the dark line?

With a good color match to the surface, the dark line will disappear when the Base coat color is sprayed on to the surface. This is done by  blending (feathering) the outer ring of the color application with the airbrush and spray process.  The dark ring appears, again, when the base coat is sanded. The sanding process  removes the hiding ability of the blend-out of the spray.  The clear coat, applied over the sanded base coat, can not remove the dark line. The clear coat must be sprayed over a “blended and without being sanded base coat”.  This will seal the feathered affect, and hide the dark line. Then the clear coat is sanded and polished. But the clear coat cannot be sanded excessively to remove the coatings, entirely.  The clear coat will then protect the colored base coat, and leave the blended area undisturbed.

Typically, this does not happen with gel coat repairs. Why does it happen on acrylic repairs?

Acrylic is a translucent material composed of a clear polymer with pigments added to achieve the desired color.  When compared to acrylic, gel coat is a resin with a higher loading of pigment.  Acrylic products are made by vacuum forming a flat sheet, heated to a high temperature, into the desired shape.  A highly loaded acrylic resin would be more brittle, and less able to be thermoformed.  Whereas, gel coat is sprayed onto a mold surface, so it can be highly loaded with pigment as long as it can be sprayed.  Higher-priced products are made from acrylic, since it is superior in properties needed for bath ware and spas compared to gelcoat resin.  When repairing a surface, a coating is used to match the color of the product.  It is sprayed over the repair filler and it’s adjacent area.  When the coating is applied to an acrylic surface, it creates a shadow at sharp, delineated edges of the coating. This is due to the thickness and transparency of the acrylic.  Even with a good repair base coat color match,  a dark halo is cast around the repair when sanded.  The only way to address the shadow casting effect, is to blend(feather) the base coat color, and apply (spray) a clear coat over it for protection and to preserve the appearance.

How to “blend” or “feather” the MMA base coat?

A single action (important) air brush operating at 35 psi (1 cfm air source) at the gun is required.  First, spray the base coat sufficiently to cover the filler area. This may take several layers (see MMA instructions).  Then open up the airbrush spray pattern to allow a higher volume of product to be sprayed. Using the correct temperature thinners and a smooth spray-out from the gun, pull the gun away (6-8 inches) from the spray surface, and feather coat the base coat around the edges of the previous sprayed area. Try to achieve a smooth surface that hides the edges of the over-spray.  If the product has sprayed a little dry and the surface around the area looks dull, wet-out those areas with straight MMA Finishing solvent from the gun in a clean separate jar. Do not over flood the area. Allow the coating and Finishing solvent to evaporate  (dry) before clear coating. This is the procedure required to create undetectable repairs on surfaces.  This spot repair technique also works with our Quick Glaze Systems.

Here is a summary of the important reasons to use Clear Coat over MMA Basecoats:

-Clear Coat seals the base coat color to retain its match and blend (feather effect) during the sanding and buffing process.
-Clear Coat is imperative for gloss and wear protection.
-Clear Coat adds depth to the acrylic look and effect matching the original surface.
-Clear Coat seals the color coat so it will not absorb detergents, dirt and elements from hard water.(most important) -Clear Coat is a necessary system component formulated and designed to be used with every MMA Basecoat application.

Other important MMA System components and tips are:

-The proper airbrush (Single action) 35 psi at the gun with a 1 cfm air source. Very important.
-Use MMA System Thinners, only. Alternative thinners will effect spray performance and final color.
-Use proper rated temperature thinners for ambient working conditions. The wrong thinner selection can cause dry spray out.
-Note: MMA Basecoats will darken as they dry. Decide on color adjustments after it dries.  Light forced heating is okay.
-Sanding can be performed on base coats, but reapply the base coat and perform the feathering prior to Clear Coating.

Here is a link to the MMA System repair instructions.


Rob Clos



Dear Multi-Tech Products;

I have a crack in the radius at the bottom of an acrylic bathtub.  Must I contract someone to install an inlay to replace the entire bottom of the bath, or is their an acceptable, easier way to fix it?  See the photo.



photo 2 photo 1

To: Roger

The use of an inlay to repair cracks in the bottom of a bathtub is always the safest way to prevent re-occurrence.   However, after saying that, there can be some latitude in recommended repair techniques depending on the severity and exact location of the crack(s).  I would say that your example, where the one crack is located right at the radius does not require an inlay.  If the crack extended into the flat area, it would need a more stealthy repair offered by inlays.

Your crack, most likely, occurred due to either insufficient support under the base, or movement in the tub.  In sufficient support allows the tub bottom to flex under weight.  This repeated flexing eventually results in crack formation.  Therefore, my first recommendation is to add structural support by injecting our 5-lb. density polyurethane foam into the base cavity to restrict movement.  It does not increase basic structural strength, but it is effective in reducing movement.  If additional structure strength is necessary, it will need to be added using fiberglass resin over the top of the weak area.  The combination of these two processes will provide the permanent repair desired.

You can refer to the blog entry, FIXING A SQUEAKY, FLEXING BATHTUB OR SHOWER, for a description of how to add polyurethane foam.  Your small crack area can be fixed using a glass fiber stitch mat with our Binding resin.  The stitch mat should be cut to a size that completely covers the crack, and extends about 3 inches beyond it.  Refer to http://www.multitechproducts.com/content/Procedures/Binding-Resin-Spa-Blister-Repair-2010.pdf for instructions on laying down stitch mat.  Obviously, this means the repair zone will be raised in relation to the normal surface.  So you should use our Poly-Filler or Poly-Paste to buildup and transition smoothly to the existing surfaces.   Since this area is at the edge of the tub bottom, it will not effect water drainage.

Roger, theoretically,  any crack on a bathtub bottom could be repaired in this manner.  However, based on the size and exact location, it could significantly diminish the ability of the tub to drain, properly.  Your case is simple, and these procedures should result in a very satisfactory repair.  Crack size and location are the determining factors whether to resort to an inlay repair.  Inlays also offer the ability for a technician to offer the optimum warranty for a repair.

After these procedures are completed, the color needs to be restored to match the other surfaces.  Refer to the procedures on bathtub crack repair for directions on preparing the surface and spraying a colored coating over the repair.  You may also choose to use our color-matched repair paste.   In cases that the bathtub has a textured surface, the surface can be duplicated using the texture additive materials described in repairing granite spas.  However, you may decide that it is acceptable to leave a small smooth area at the repair, if it is not noticeable.

I hope this information helps you to resolve your problem.  Please call if you have further questions.


Rob Clos

TO:  Multi-Tech Products

I have a spa that has developed crazing in the acrylic surface.  See the photo.  How can it be repaired?



TO:  John

Repairing a large, crazed area is complicated, since it requires removal of the affected acrylic surface.  A hammer and chisel is normally required to remove this material.  Be carefult not to punch a hole completely through the wall structure.  Then our Acrylic Filler and color-matched coatings are required to refinish the area.  Refer to the procedures for repairing spas with the Quick Glaze system.  Often, the cause of crazed acrylic is the presence of trapped styrene in the polyester resin backing system.  So to prevent re-occurrence, you should use a heat gun to evaporate any residual chemicals in the exposed FRP prior to starting to apply the acrylic filler.  See our explanation of crazing in the website, http://www.multitechproducts.com.

Remember that crazing affects only the appearance of the surface.  It will not result in water leaking through the structure.  In some instances, where there is minimal crazing, it can be softened with heat.  Using a heat gun, you would heat the surface up to about 220ºF, which will cause the crazing to relax and reduce the severity of the micro-cracks.  One acrylic manufacturer suggests that wiping the area with a paste wax, or equivalent, sometimes works to fill in the micro-cracks to make them less visible.  You could try some of the MTP paste repair kits.  Using a cover on spas is very important in avoiding these and other surface issues.

I hope this has been helpful.

Rob Clos

TO: Multi-Tech Products Corp.

I have a new acrylic spa that has brown spots all over the acrylic and the side panels.  What caused it, and how do I get rid of them?  Please see the attached photos.






To: Kevin

The brown spots appear to be diphenylmethane diisocyanate (MDI), which is the most common isocyanate used in polyurethane foam.  Polyurethane coatings and foam are used both as reinforcing and insulating materials in spas.  MDI is the “A” component in the two-part resin system, and has an amber color.  Part “B” is an organic alcohol, and is normally white.  The MDI will continue to get darker in color as it oxidizes with age.

The MDI can be on these surfaces due to over spray during the manufacturing process.  Since it can take hours for the color to appear, the manufacturer may not have seen it during their cleaning steps.  Also, if the workers have it on their hands and tools, and touch these surfaces, it will leave a residue.  This occurs when factory workers are not diligent in maintaining cleanliness.

There are multiple considerations for removal.  If it is foam ( i.e. mixed parts “A” & “B”), sanding may be required.  Also, if the chemical has been absorbed into the surface, it may require sanding.  Sand in a circular motion with 400 grit followed with 600 grit wet/dry sandpaper using water.  The acrylic surface can be renewed after removal by following our “Buffing and Polishing” procedures and/or video.  Polishing with medium grit compound using a 2500rpm electric buffer will bring back gloss.  Polishing would not be required on granite textured surfaces.   See our procedures for granite repair for more details.  If it is on the artificial wood or plastic side panels, sanding may remove the color making it necessary to re-stain or paint.  You can test in a non-conspicuous area.  You can inquire with the manufacturer about their repair recommendations.

If you desire to avoid sanding and surface renewal, you can test the effectiveness of using solvents to remove the foam chemicals.  Start by using plain water with about 10% ammonia and some household dish detergent.  Water reacts with isocyanate to form non-toxic carbon dioxide gas.  If it fails, try isopropyl alcohol, which has low risk.  Again, if unsuccessful, try standard paint lacquer thinner.  Finally, try a commercial xylene solvent, being careful to use a minimum quantity.  When finished, the xylene should be completely cleaned from the surface using water and the alcohol.  Xylene is not recommended for long exposure to acrylics, since it will lead to stress cracking.  The acrylic used for spas is cross-linked to impart superior chemical resistance compared with standard acrylic products.  So be careful to remove all excess solvent.

I hope this helps to resolve your problems.

Ken Wolfe,

Consulting Chemical Engineer

Questions regarding causes of fisheyes in repaired bathtub surfaces have arisen over the years.  These can be created from in-plant repairs, or work performed by repair professionals.  They can occur in both MMA and Quick Glaze repairs.  One source for these problems is from oil or moisture in the spray. Following is a description of how they occur.

Rob Clos

1)  Oil and high moisture in the air lines:

It is common for compressed air source lines to carry a high concentration of oil and moisture.   Moisture is created from condensation as pressure and temperature are changed in the compressor.   This is especially true in high humidity regions.  With water present, the lines and tank reservoirs can also rust, carrying oxidation through the lines to the airbrush or spray gun onto the repair, which changes the spray color.  Oil  can impart a color  to the spray, also and it can cause fisheyes in the sprayed surface.

Water traps simply do not work efficiently to remove these liquids.

untitled1 (2)

The picture above shows a common  water trap, which our experience shows  are ineffective.  Regardless of the size or quality, it will  not remove 100% of the water or liquids.

untitled2 (2)

An industrial grade desiccant filter tower (shown above) works best to remove all foreign liquids.  This product is recommended, and can be installed at the air supply source to serve the repair area in manufacturing plants.

Contaminated compressed air enters the single tower desiccant dryer and flows downward through a bed of silica gel desiccant. As the desiccant removes the water and becomes saturated, the color of the desiccant life indicator turns from blue to white.  The air then flows through an integrated dust filter up to the outlet port of the dryer. The air is dried to a -40°F dew point and is ready for use.

The following picture shows an in-line desiccant air filter for portable use, especially for repair professionals, and where little moisture is present in the lines.   MTP sells these units. (click on picture)

untitled3 (2)

With the air lines drained of moisture, a desiccant air filter stops moisture and oil through to the repair surface. This filter will  prevent fisheyes from occurring in the repair surface during the spraying process.

It is common to have so much moisture in the lines that the water is visible when the air pressure is released through the lines.  Therefore, drain the compressor often so there is no visible sign of water.  Also, drain air source lines and hoses. With the compressor drained and air pressure recharged, open up and run dry/clean compressed air through the lines until the lines and hoses are cleared of moisture. Now attach an inline desiccant filter before or after the regulator and you will see a difference in the coating application performance.

TO:  Multi-Tech Products

I am a professional spa surface repair contractor.  I frequently have to repair spa blisters.  Your procedure works very well, but it is more detailed.  Can I simplify the process by just injecting acrylic resin or an epoxy into the blister at the edges to create adhesion to the FRP?  This will save having to grind out the acrylic and causing a larger area to repair.

Thanks, John


Unfortunately, this method will not yield long-term repairs that you can apply a warranty to.  In order to achieve the desired result, a permanent repair that will not result in a call-back, we strongly recommend that you use the process that has been developed for spa applications, and has proven to work long term.  Since blisters are filled with a fluid, mainly water, the fluid will interfere with the polymerization of acrylic monomer resins, polyester resins, and epoxies. Even if there were a way to rinse out the fluid inside the blister, the root cause of the blister (wetness in the FRP) will still be there; not addressed, the same blistered area will have adhesion, wetness and continued bulging issues.

The method in question is often called the “poke and roll-out” procedure.  The blister is heated with a heat gun until a small hole can be punctured into the top surface.  The fluid is allowed to drain, and with the acrylic still soft and pliable, it is hopefully rolled or pushed back into its normal shape.


This photo shows a blister opened up.  Notice the fluid.  You can also see that the acrylic was not adhered to the FRP.

Theoretically, resin or adhesive of some type is injected into the blister to glue it into place.   This is not a permanent professional repair, since the same blister or a new blister will form when the hole(s) are filled, or if the hole(s) are left open, it will leak or ooze the fluid that will discolor the surrounding surface. Simply put, the root wetness needs to be addressed.




This photo shows an improperly repaired or rolled out blister that has the fluid, which was not removed during the repair,  seeping out of the repaired surface.




However, it could be a temporary fix if that serves a purpose. Normally, the manufacturer wants a permanent fix with a warranty.

Other problems can occur with heating a blister and laying it flat like:

-The acrylic will eventually crack causing a sharp or dangerous edge.

-The acrylic can split and melt during the process.

-The blister will not lay flat due to delaminated FRP behind the blister.

-Often large amounts of calcium builds up behind the blister keeping it from laying flat as well.

-All the above is the reason why a resin or adhesive will not adhere, let alone cure properly.




This photo shows acrylic removed from blister, and a perfect example of the laminate shredding as the blister forms.


A proper fix would consist of removing the blister, the fluid, force drying the FRP, sealing the area with the proper resin and matting, followed by the filler and color process.




This photo shows using a high temperature heat gun to dry the blister area.  Extreme heat must be applied to remove 100% of the fluid.  You will notice that the wetness extends deep into the FRP.




After the wetness is eliminated with heat, seal with our Binding Resin while saturating a layer of fiberglass matt.  The resin and matt thickness will eliminate a step in the filling procedure, making the filling process more efficient.



Following these instructions will assist in the prevention of repair failure due to fluid reformation in the repair zone.


You can also read about how blisters form:


This page will give customer relations tips, as well as other helpful information:



Rob Clos


There is no simple answer to this question.  Many factors need to be considered to develop the best, successful bid for a spa surface repair job.

First, is the request coming through the spa manufacturer, and is it under their warranty?   If so, they may have a standard fee schedule that applies.  Inquire about it.  Warranty repairs normally require the contractor to provide a guarantee of performance.  Higher prices are common on warranty jobs.

Additional information needed is:

  1. Does the job only require a cosmetic fix, or does it include a structure repair prior to the cosmetic repair?  Additional costs are required when a structure repair is necessary.
  2. If it is a warranty job, is the request simply for an evaluation visit to determine magnitude of the damage or is it for the actual repair.
  3. How far do you need to travel to get to the site?  Are other professional repair contractors closer, and are they bidding the job?
  4. What are the prevailing hourly rates for professional service contractors in your area?  Some contractors use job rates based on the magnitude and type of repair.  Obviously, it must take into consideration the time required to complete it, and a fair hourly wage.
  5. How does your skill level compare to your competitor’s?  There is an opportunity to earn more money if your skill level or materials provide a competitive advantage.  You can quote the cost based on what an average skill level contractor would charge, but your quicker completion and/or higher quality materials results in more profit at the same cost to the customer
  6. Would it be cheaper or wiser for the customer to replace the spa rather than repair it?  Reputable contractors will advise customers, accordingly.
  7. What is the defect being repaired?  Is it a blister(s)?  Is it a crack(s)?  Is it crazing?  Make sure you know what is involved before you make a quote.  Also, make sure the customer understands the scope of your quotation.
  8. Is the surface acrylic, or some other material?   Does the acrylic have an ABS reinforcement and/or fiberglass-polyester resin?
  9. Do you have all of the required materials for the repair, and what did they cost?   Typically, material costs are a small percentage of total cost for a repair.  These can be minimized when components are purchased in bulk quantities rather than single use kits with each required component.
  10. What color is the spa?  Granite-like acrylic colors are easiest to repair.  Marble patterned spas are the most difficult to repair, since it takes an experienced and skilled operator with an airbrush to duplicate the pattern so it is undistinguishable.  Some non-acrylic surface spas require special materials to repair.  Pearlescent colors also require special coatings to duplicate the appearance.


Visit http://www.multitechproducts.com/pages/Blisters.html for a technical discussion on what causes blisters.

Visit http://www.multitechproducts.com/categories/Instructional-Videos/  to order a copy of the DVD to learn everything on repairing spas.


1)  A 2” diameter blister in the lounge seat of a Granite colored spa.  The customer has drained the spa prior to your arrival.

–         Total time to prepare and complete repair by an average skill technician =  3 to 4 hours

–         Cost of materials ~  $50  (based on buying one single use repair kit)  Repair professionals that perform frequent spa repairs buy individual components, and can reduce their material costs for a single job.

–         Quotation Price ( including maximum of 1 hour travel time) = $250 to $300

Comment:   A similar repair on a Marble pattern spa would require an additional one hour, and the total Quotation price should increase to $300 to $350..

Additional blisters in the same granite spa should be quoted at a reduced cost.  The increase for the second blister should be 50% ($375), and the third blister should be an additional 30% ($450) of the first blister.  Reasonable judgment should be used for cases with numerous blisters.  It may get so costly that it would be cheaper to replace the spa, particularly if it is a portable one.

GO TO  http://www.multitechproducts.com/pages/Procedures.html for specific directions on repairing a spa blister with the MTP Binding Resin.

2)  A 6” long crack at the rim of a Granite colored spa.  The spa has been drained by the customer to provide a dry working space.

–         Total time to prepare and complete repair by an average skill technician = 2 hours

–         Cost of materials ~  $50 (based on buying one single use repair kit)

–         Quotation Price ( including maximum of 1 hour travel time) = $200 to $250

Comments:   A similar repair on a Marble pattern spa would require an additional one hour, and the total Quotation price should increase to $250 to $300.

Additional cracks in the same spa should be quoted at a reduced cost.  The increase for the second crack should be 50% ($300), and the third crack should be 30% ($375) of the cost of the first crack.

GO TO  http://www.multitechproducts.com/pages/Procedures.html for specific directions for a cosmetic repair of a spa crack.

3)  A 2” diameter hole in a spa, which requires rebuilding the FRP reinforcement.  Depending on what caused the hole, there could be spider cracks emanating from the circumference of the hole.  These will increase complexity of the repair, and the cost.

–         Total time to prepare and complete repair by an average skill technician = 4+ hours

–         Cost of materials ~  $30 for FRP structure materials and $50 for cosmetic repair kit

–         Quotation Price ( including maximum of 1 hour travel time) = $400 to $450

GO TO  http://www.multitechproducts.com/pages/Procedures.html for specific directions for a structural repair of a spa.