Tag Archive: spa discoloration

TO: Multi-Tech Products

I am a spa dealer with a customer that has reported a problem on the surface of his acrylic spa. The attached photos show marks appearing on acrylic shell surface. They were in the area around the filter canister, and looked small stone chips that you get on the front end of an automobile. The customer is one off our best on chemical management and takes great care throughout the year to service his tub.

Can you take a look at the pictures and give me your thoughts on the potential cause?  I have never seen this type of issue on a shell. It’s within about a 12″ area within the filter housing but doesn’t seem to be anywhere else.  Could it involve a previous surface repair?




To: Tony

The pictures indicate that this is another example of acrylic pigment fading and/or topical blistering due to exposure to a strong sanitizing agent (oxidizer).  Although rare, we have seen several similar examples over the years.  One acrylic manufacturer has even been able to duplicate this phenomena in the lab using “Tri-Chlor” tablets.  Someone has created this problem with pool chemicals either by using the wrong chemical, or using too much of an approved chemical. The appearance is consistent with what we have experienced with Tri-Chlor.


The logical mechanism of your example is that a very strong oxidizing chemical (probably in solid form) sunk into the water, fell down the vertical surface creating surface damage as it sunk into the bottom of the foot well or seat.
Acrylic is by far the best material for spa construction, and withstands many harsh conditions, but it is not perfect.
Although this type of damage is rare, it suggests that the industry should increase it’s communications on chemicals to avoid, and how to use the recommended ones.  There is evidence of extreme chemical dispersion into the filter housing, and it reached a very high localized concentration.

From my observations from the photos, I see:

1. A peeling effect of acrylic. This could happen where a tablet would set for a period of time in one spot on the acrylic.

2. A scratching effect. This can be where a large round tablet would float and roll around creating damage to the surface. Or the scraping of chemical damaged or repaired acrylic.

3. Rust in the affected areas. This would be from iron in the water reacting with the oxidizer, and depositing on the porous surface created from the chemical damage.

4. Attack on the silicone in this area also suggests the presence of strong chemicals in the same area.

5. Bleaching of the black filter fixture is an additional indication of chemical oxidation.

If there is a repair in this area, the peeling would be of a clear coat. However, the same chemical damage, described above, most likely would have caused the damage to the repair.  A pre-existing repair could only be determined by inspection by a repair pro that has experience with chemical damage.

The good news is that it can be removed by sanding through the oxidized layer (chemical damage) down to good acrylic.  Then it can be buffed and polished to a higher gloss. It will depend on how thick the oxidized layer is, which depends on how long the acrylic surface was exposed to the high concentration of oxidizing chemicals.  Hopefully, the discoloration is only “skin” deep. From my experience, it looks like it is less than a 1/16″ (1.6mm) deep.

The colors could change as you sand, and become darker or lighter.  It would be unpredictable based on the pigment mixing through the sheet. That should not be a big problem since it is in the filter area where it is hidden from view.   The gloss would be maintained through the proper buffing and polishing process.  If there is a repair, you should find a light-colored filler under the repair coating.  In this case, depending on the repair filler depth, it would need repair including a new coating and clear coat for protection. It could have been a surface repair that just covered up a light surface issue, and may buff and polish out to a normal appearance as well.

Here are other examples of the chemical issues and more explanations of causes. Follow the links in the articles to the proper remedies:

To repair the surface, sand with a progression of 220, 320, 400 grit sandpaper. Finish with 600 grit wet/dry sandpaper. Then to recover the gloss, buff using a high-speed (variable to 2500rpms) with a good quality buffing pad and medium grit buffing compound.
We have a video demonstrating the entire procedure, and it can be down loaded.



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)