To:  Multi-Tech Products

I have a spa that is very new (about 2 weeks), and it has already developed some yellow stains on the surface.  Can you tell me what caused it, and how to remove them?  I have attached a photo. (below)                                                                                                         Thanks,      John


To: John

Although it is difficult to diagnose a problem with very little information on how the spa is used and maintained, this issue looks like the same problem we have seen in the past.  Almost 100% of the cases involving discolored acrylic surfaces are caused by chemical attack.  We have seen many examples over the years where spa owners have used the wrong water treatment chemicals or water fragrances, and it resulted in damage to the acrylic surface.  It doesn’t take long exposures to cause the discoloration.  The most common culprit in recent history is the chemical “Tri-Chlor” (Trichloroisocyanuric acid).  This product is sold in tablet form for swimming pool water treatment. It is the most popular swimming pool chemical sold in stores like Home Depot and Lowes. Trichlor is essentially a combination of Cyanuric Acid (CYA) with chlorine.  Dispensers are used in pools to slowly dissolve the tablet.   The dispenser floats on the water surface, but in a spa, it normally moves to the water’s edge and rests along the spa surface due to water agitation.  So, the acrylic surface close to the tablet is exposed to a very strong acid, which bleaches the pigments.

The problem has occurred enough to cause one acrylic manufacturer to conduct lab testing to verify the problem with Tri-Chlor.  Their testing has proven that Tri-Chlor can cause bleaching of acrylic pigments.  The following photo shows another example of this chemical attack from Tri-Chlor that is from their testing program.  The color of the stain depends on the acrylic pigmentation, and influences from other chemicals used in the water.

Bilo photo

Their testing results prompted them to publish a technical bulletin for their customers, and to produce a label (see below) to be applied to new spas.  The label warns the spa owner to avoid Tri-Chlor, and the label should be used by all manufacturers and dealers.


Spa owners should refer to their product operation manual or consult with a qualified dealer or the manufacturer to learn recommendations on proper water treatment chemicals to use, and what products should be avoided.  Di-Chlor (Dichloroisocyanuric acid), in powder or granular form, is a weaker acid and a commonly recommended product for hot tubs and spas.

The good news is that, in most cases, the discoloration doesn’t penetrate too far into the surface.  This means that our standard procedures for sanding, buffing, and polishing can be used to remove the top surface layer to recover the original spa coloring.  This procedure is at:

Click to access BUFFING%20AND%20POLISHING%20NCINSTBP-v.pdf

I hope this helps you, and thanks for trusting us for help.

Regards,                                                                                                                                                 Rob Clos