Tag Archive: repair

To:  Multi-Tech Products

The pedestal base on my vitreous china bathroom lavatory is broken into two pieces.  Do you have a product that I can use to glue them together?




To: Tom

Yes, you can use our Poly-Filler to glue vitreous china together.  You will need to prepare the surfaces to be joined by sanding with 220 grit wet/dry sandpaper.  It will set in about 3- 5 minutes.  If you need more time, you can refrigerate the Poly-Filler prior to use.  This will double the working time.  When completed, there will be a line at the joint.  This can be hidden using our MMA repair kit, which can be supplied in a color-matched coating.  The entire pedestal would need to be refinished to achieve a uniform color.  You will need to let us know the manufacturer of the lavatory, and their color name.  You will follow the directions shown in the MMA procedure, which is available in our website.


Rob Clos


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
Do you have a kit to repair ABS pans on a hottub?  We manufacture spas with a base (pan) made from ABS to protect the bottom frame from the outdoor weather, etc.   Sometimes, it gets damaged in transit to the dealer or customer. It is not cost effective to send an entire new pan, and some of our dealers have asked for a repair kit. I would appreciate any help.
Thank you,
TO: Daniel

Yes, we offer a very good solution to your need for a kit to repair minor damage to ABS pans on the base of a portable spa. The kit contains the necessary components and procedure to repair moderate cracks, skid damage, and tears. You can read about the procedure by going to:


You may order by calling Customer Service at 800-218-2066.
Rick Borden
Customer Service Dept.

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 am a repair technician; I have inspected a cultured marble whirlpool with a 4″ vertical crack on the tub. The crack is on the bottom radius around 12″ from the drain. The tub has leaked through due to the crack.  Do you have a method/product that we can use to repair cultured marble and guarantee that the tub would no longer leak.

Any help you can give us would be greatly appreciated.  We look forward to hearing back from you.  If you have any questions, please don’t hesitate to contact us. Otherwise, have a wonderful day.

All the best,

TO: Maureen

You describe a thermal stress crack created by the shock of hot and cold water.  I have seen this type of crack go completely around the perimeter of the tub.

In order to repair it, and prevent future propagation of the same crack, you need to drill a hole at each end of the visible crack.  Be sure to drill through the entire depth of the tub with a ¼ drill.   Then grind a deep slot into the structure along the entire length of the crack. Just grinding an 1/8th inch groove in the top surface is insufficient since it will continue to crack. This is because the crack goes through the complete structure. The surface will continue to expand and contract as it is exposed to hot and cold water. This expansion and contraction will lead to a new crack directly through the repair. Cultured marble bathtubs aren’t reinforced with glass fibers, so they have less strength to resist failure from expansion cracks.   If there is access to the back of the bathtub, you should attempt to add reinforcement using fiberglass mat.  The combination of reinforcement on the back side and grinding the crack open through the structure will provide the best permanent repair.

The fact that this is a relatively short crack improves the probability that this crack can be satisfactorily fixed.

If there is no way to get glass on the backside, aggressive grinding and heavy glass reinforcement from the top side will achieve a good repair. The repair procedure should be:

  1. Grind the crack to create a recess in the top surface along the crack wide enough ( probably 2 to 3 inches) to accommodate the fiberglass mat.
  2. Using a heavy duty die grinder, grind a slot channel that would hold a recessed layer of heavy duty stitch mat. The stitch mat would be about an 1/8th of inch thick. A slot grove would need to be ground ¼ inch deep to hold the mat and the resin while leaving enough room for the poly filler after the mat and resin application. The slot groove would not go through the structure, but leave enough structure to bond to. The slot groove would hold the width of the Stitch mat to straddle the crack a little over an inch on each side of the crack. If the crack is 1/16th inch by 12 inches. The slot groove should straddle the crack by 1 ¼ on each side and extend past the end of the crack the same. Total groove over the crack 2 ½ inches x 14 ½ inch long by ¼ deep.
  3. With the groove and the crack prepared to except the fiberglass application, apply catalyzed Iso resin to the glass first and place it in the groove.  Smooth out and remove bubbles.
  4. Force cure with a heat gun.
  5. Fill with Poly-filler.
  6. Use our Quick Glaze system with the Clear Coat to achieve better color match and appearance.

You should charge your customer between $350 to $450 for this best-practice job, and it should take 3-4 hours to complete.

Hope this helps.

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

To:  Multi-Tech Products Corporation

I have recently purchased an acrylic spa, and the salesman strongly recommended that I keep a cover over it when it isn’t being used.  Why is this?



To: Jim

There are several valid reasons for using a cover on a spa when it is not in use.

The first is obvious.  A properly designed and insulated cover greatly improves the energy efficiency of the unit.  The cover slows the rate of water evaporation, which removes heat from the water remaining in the spa.  Also,  evaporation control will result in maintaining proper chemistry of the water, which will reduce usage of the chemicals needed.  These facts help control operating costs of the spa.

It is probably understood by everyone that ultra-violet radiation from the sun is deleterious to all plastic materials.  Acrylics have very good resistance to sunlight, which is why it is commonly use for windows, skylights, aircraft canopies, etc.  But, it is not perfect.  Although, we may not understand all of the ways sunlight harms polymers, we know we should minimize exposure.  So, we strongly recommend using a cover when the spa is not in use.

Another issue regarding acrylic spas is the formation of cracks or crazes.  Both of these phenomena involve failures due to excessive stress.   First, lets review the construction process.  Acrylic spas are produced by heating and forming a flat sheet into a mold shaped like the spa.  The sheet is stretched from the top rim into the bottom of the mold cavity (foot well of the spa).  Obviously, several things happen.  As the sheet is stretched, it becomes thinner, and weaker.  The acrylic thickness in the foot well is normally in the range of .030″.   This stretching imparts stresses, and some of the stress remains after it is cool.  Then most “spa shells” are reinforced with polyester resins containing chopped fibers of glass.  So it becomes a composite structure with each material having unique properties.  Stresses exist throughout the spa shell, and arise from the thermoforming and reinforcing steps, and from the weight of water and people using the spa.  Material strength is effected by temperature and exposure to chemicals.

Crack(s) in the acrylic layer occur when stress exceeds the material strength.   Stress can be based on thermal changes or induced mechanically.  If covers are not used properly, the surface (acrylic) is subject to wide swings in temperature from  sunlight during the day to very cool night temperatures.  This heating and cooling can result in fatigue failure cracks from repeated expansion and contraction.  The mechanical stress created by the combined  weight of water and people also can lead to cracks.  Therefore, these need to be minimized using adequate support devices under the shell – particularly under the seats.  Increased temperature reduces material strength, so if stress and temperatures are high, a crack can occur.  Since stress is typically highest at the spa rim, cracks generally start there, and propagate into the center.  The use of a cover will minimize effects of temperature, and reduce probability of crack formation.  See the picture for an example. of cracks at the rim.







Another type of crack phenomena is crazing.  A craze is different from a crack in that it can’t be felt on the surface, and it may be able to support a load.  Many studies have demonstrated that two conditions must be present for stress crazing to occur on an acrylic spa surface.  They are 1) high stresses within the acrylic and the 2) presence of a stress-cracking liquid or solvent. The presence of only one of these conditions does not cause crazing.  Both must be present.  Crazes form at highly stressed regions.  Stress can occur due to thermal or mechanical forces.  Crazing occurs mostly in amorphous, brittle polymers like polystyrene (PS), acrylic (PMMA), and polycarbonate (PC).  The acrylic used for spas is a specially designed to withstand the normal spa environment, but it will fail under harsh conditions.


Crazing appears as very small micro-cracks on the surface (see photo).  The stress pulls apart the tightly coiled polymer chains in the material. This condition makes it easier for liquid molecules to penetrate the molecular structure of the acrylic and diffuse throughout the polymer chains.  The crazing mechanism is the stress cracking molecules act as a lubricant, which allows the polymer chains to separate from one another when stressed, creating very small cracks. Depending upon the amount of stress and the aggressiveness of the chemical agent, the small cracks continue to grow in size.   Since the strength of the acrylic declines with temperature,  the use of a cover will minimize thermal stress, and lower the risk of crazing.  Since the over-riding cause of crazing is the presence of a chemical, it shows the importance of avoiding the use of stress-cracking chemicals on the acrylic surface.  Only use approved chemicals for cleaning and maintenance of water chemistry.

Finally, many manufacturer’s warranties are voided when covers are not used.

For more information on crazing, please click on the link below:

Tech Bulletin Stress Crazing.pdf

I hope this helps to explain your question.

Ken Wolfe

Consulting Chemical Engineer

Fixing a crack at a spa jet

To:  Multi-Tech Products

My spa has a crack at one of the jets, and water is leaking through. Will your acrylic granite repair  kit permanently seal the crack?  How can I remove the jet to make the repair?  It seems to be anchored very tightly.



Jet Crack






To: Dennis

Obviously, you have a crack in the acrylic surface, and you report your spa has evidence of water leaking into the rear area of the shell.  Although it might be possible for the two events to be related, generally they are not.    The reason that most cracks in the acrylic surface do not lead to a water leak is that spa shells are constructed using at least two layers of different materials.  The acrylic surface is only used to provide the attractive color and effects.  This outer shell is produced from an acrylic sheet that is heated and formed (stretched) by vacuum into a mold.  The original sheet is only about .125″ thick, so there are areas in the bottom where the acrylic thickness is between 0.020″ and .030″ thick.  So most spas are sprayed with a polyester resin filled with chopped fiberglass strands (FRP) to provide the strength required for the shell.   Also, most manufacturers then spray a coating of a high density polyurethane foam on top of the FRP for insulation.  The FRP reinforcement is very strong, and it is adhered to the back of the acrylic.  It would be very unusual for this FRP structure to crack, which would be necessary to lead to a water leak.  Although the acrylic surface of spas are known to crack for various reasons, water is still contained by the FRP structure.  If you can actually see water leaking behind the crack, it would be one of those one in a million situations.  However, since there is a hole in the shell at the jet, it might be possible for the water to find a path through that hole.  Typically, silicone sealants are used to prevent leaks at the jets.

The most common source of water leaks is the plumbing.  These leaks are often difficult to find due to the insulation in the spa cavity, especially when the cavity is completely filled with polyurethane foam.  You may need to get a spa maintenance expert to locate and repair a leak in the plumbing.

However, to answer your question about the acrylic repair, our granite repair kits are designed for this purpose.  Please refer to our website, http://www.multitechproducts.com/pages/Procedures.html, for a description of the materials and procedure for the repair.  The granite repair kit will repair the crack up to the jet, and without removing the jet.   If the jet needs to be removed, you need to consult with a spa mechanical repair contractor or the manufacturer.

I hope this will answer your questions.

Rob Clos