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