Tag Archive: Acrlic

To: Multi-Tech Products:
This repair (see pics) was done about 3 months ago and is now turning cloudy. What could be the cause ?
I’m thinking a reaction to spa chemicals?

IMG_1621 IMG_1622

To: Doug,
It is difficult to determine the exact cause of this problem without knowing some of the history and common water chemistry maintenance practices used on the spa. There are least two causes. Information on them follows.
Here is information on common chemical damage to acrylic spas. I don’t suspect this to be the cause, since issues would occur at other areas of the spa – not just at the repair.
The next article describes what can happen to our Clear Coat if it is exposed to moisture before complete curing – normally requiring 24 hours..
I think this is the most likely cause in your case. I am assuming you used the MTP Clear Coat to finish the repair, since we believe it is an absolute necessity to achieve a quality repair.
Three months is normally the time window that post cure repair issues show up.
The repair could certainly be damaged by strong oxidizing chemicals (e.g. tri-chlor). However, Tri-Chlor would also cause fading in the acrylic in other areas of the surface. So, I don’t believe tri-chlor exposure is the culprit.
If the problem is simply premature exposure to water, it is an easy fix. You just need to remove the repair coating, and respray.
I would recommend sanding with 400 grit sand paper to remove the top coating. Light sanding can remove the damaged Clear Coat while preserving the Color Coat. At least, it could limit the need to spray more of the Color Coat.
After completing the repair, it is very important to keep the repair zone dry for a sufficient time to allow the coatings to completely cure.  Exposure to any form of water may effect the coatings.   If the repair was below the water line, wait for at least 24 hours before refilling the spa.
This communication addresses the post-repair cure procedure, and how to place the cover to protect against water getting on the repair zone.
Hope this helps,


To: Multi-Tech Products

I own a spa service company, and I have a customer with a unique problem. He owns a large yacht, which had a spa installed several years ago.  It has been abused by failing to keep it covered when not being used. Also, it may have been exposed to the wrong water maintenance chemicals over the years. It is extremely faded, and has crazing and cracking in some areas of the acrylic surface. Since it is still functional, and would be expensive to remove and replace, the customer is inquiring if there is a simple way to renew the surface color and fix the cracks. He is asking about painting with a pool paint or Zolatone® to match the granite appearance. There are a lot of jets in the spa, and some would be very difficult to remove, if a spraying operation was required. What do you think?



TO: Bill

I agree with you. The spa is crazed and cracked mainly due to over exposure to the sunlight (i.e. no cover in use).  Also, the use of strong water chemicals like Tri-Chlor was probably the major factor in the fading, although the excessive UV exposure contributed.  Acrylic sheet manufacturers have confirmed that Tri-Chlor will fade the pigments in acrylic spas and should be avoided for use in maintaining hot tub water chemistry.  Tri-Chlor is intended for use in swimming pools.
First, Zolatone® is a brand of hybrid lacquer paint, and will peel from a wet environment surface rather quickly.  This product, although speckled in color, is intended for top-side and non-wet areas in marine, auto, and industrial applications. It should not be used below a spa water-line, in tank or wet bilge areas. Even clear coated, Zolatone® will delaminate when painted onto constantly wet and high moisture environments.
So, a spa/hot tub refinish is out of the question. In fact, there is not a coating available that would provide a “quick” fix to renew a crazed/cracked acrylic hot tub surface long term. This even includes swimming pool and bathtub coatings, as well as marine finishes, and let’s not forget gel coat as paint. The reason is that the acrylic surface will continue to crack, craze, expand and contract under the applied painted coating. The cracks will simply come through, along with rapid delamination (peeling) of the coatings. These paints are just not made to withstand heated water, constant moisture exposure or chemicals.  These factors will cause  bubbling and peeling of the coating.  However, they do work well when the surface stays dry.  Moisture must dry quickly from these coatings in order to stay pristine, and adhered to the resurfaced substrate. They work well for resurfacing bathtubs. The tub is filled (gets wet), drains (dries) and the surface will continue adhered to the substrate.  Overuse, or a dripping shower head or fixture will result in constant wetness and the paint will fail.  Furthermore, pool coatings or marine anti-foul paints have an application life of only about 3 years. Therefore, these coatings would have a short life expectancy on a spa in a spa application.  Under spa conditions, we have seen these types of coatings fail within a few months from application.

Since there was no commercially available means to meet this need, we developed a process that works, and performs well under normal spa and wet conditions.  It is a special resin embedded with glass fiber reinforcement. The reinforcement along with the high performance resin adheres to the original spa structure, and creates a barrier surface with minimal expansion and contraction. The system is laid-up by hand and will give an expected life of 15 to 20 years. We developed this system specifically for spas, tanks and challenging wet areas as a economical method to extend their life.  It is our FiberGlass Reinforced Lining (FRL) system. It adds a new reinforced, white colored layer on top of the existing surface or prepared structure. You can see the system components and procedure at the following link. It is contained under the Technical tab in our website.

Your application doesn’t require removal of the jets or other fixtures, since the crazed areas on the subject spa do not appear to be causing severe peeling or delamination of the acrylic layer.  It would not be necessary to remove the acrylic or repair the areas before applying the FRL system.  The finished FRL layer will be about 1/8” thick, so it will contribute some additional strength.  In other words, it is a structural finish.  Since the FRL final color coat is white, it will provide an attractive textured finish and appearance that hides the underlying fiberglass throughout the FRL application.  It is easy to clean, and is resistant to fading by sunlight.    With proper water chemistry maintenance, it will retain its gloss.  It has proven to add long term life to spas in these high end settings where replacement was not an easy option.

I hope this helps. Our Customer Service Department will help you in ordering the proper kits and materials to do the job.

You can see the FRL components at:


Rob Clos

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