Archive for August, 2014

TO: Multi-Tech Products

I recently completed a cosmetic damage repair on a shower wall surround. I used my standard repair filler, color coating and methods. The repair failed as the filler fell out of the prepared area while the coating bonded well. I remember not recognizing the material but proceeded with my usual methods anyway. It was a small crack and the structure was solid. When I used my grinder to prepare the crack, I found a material with two or three layers. It had different colors to the layers. The thin bath ware top white color, a pure white plastic like layer, and the structural layer similar to a composite reinforced structure. The structure layer did not smell like fiberglass when I ground into it some. What do you think it was, and what is the proper way to repair it so it will hold?

Dear Steve;

It sounds like you found a product made from what is called “co-extruded” in the industry. This material is less expensive than traditional materials and is becoming more popular in economy products. You will find them at your local Home Centers. Manufacturers continually seek materials and processes to lower manufacturing costs. Historically, most bath tubs and shower products have been manufactured from cross-linked, cast acrylic sheet or gel coat. Both are reinforced with FRP (polyester resin with embedded glass fibers). Cast acrylic sheet is the most expensive acrylic on the market, and the cross-linked variety was specifically engineered for bathtubs and spas. Therefore, it has the best balance of physical and chemical properties for these applications.

The co-ex product you experienced is made by extruding a two-layer sheet – the top is a lower molecular weight, un-cross-linked acrylic, and the bottom is ABS (acrylonitrile-butadiene-styrene copolymer). Typically, the sheet is .125″ thick with the top layer being about .025″ and the bottom layer about .100″. When formed into bathtubs or shower walls, it can then be reinforced with FRP, polyurethane (PU), or have no reinforcement. PU may have glass fibers in the resin, and it can be a high density foam or a smooth coating. Sometimes it is reinforced with fiberboard strips or panels.

You mentioned there was an odor when grinding the structure. This is usually the best way to establish identity of a polyester resin. Simply use sand paper (320 grit) to sand the material in question before proceeding with a repair. Grinding may be required to expose the different layers. Polyester resins will have a distinct sweet smell with sanding or grinding friction. After the sanding test to confirm that it is a polyester, it is safe to use similar polyester resins for filling and reinforcement purposes. If there is no polyester odor, use our binding resin for reinforcement. It is formulated to adhere to PU and other plastic surfaces.

We would also recommend verifying the structural strength and integrity of the repair zone before applying the color coatings. Be certain there is no movement or stress to the area when hand pressure is applied. If there is, remove any filler and start over with the repair.

When repairing the cosmetic surface of bath products made with “co-ex” , our materials and processes work well. Our MMA or Quick Glaze systems perform very well as the color coating. In fact, since the lower molecular weight acrylic is not cross-linked, coatings adhere to it better. However, compared to polyester resins, it is more difficult to achieve adhesion on ABS or PU. So, you should not use Poly-Filler or any polyester resin based crack or putty fillers. The deceiving factor is that most polyester fillers will initially bond to the material for a short period.   However, after time it will lose its adhesion to the ABS or PU, and will break away from the repair zone. Our acrylic filler should be used for this application.

I hope this provides you the necessary information to perform the best repair.  If you need more help, please call or email.


Rob Clos

To:  Multi-Tech Products

I am a new surface repair tech, and I am having difficulty getting a good cured repair on gel coat bathtub repairs. The surface remains tacky too long, and doesn’t achieve a satisfactory hardness. What am I doing wrong?

The most common problem repair technicians have is “a tacky surface” after the sprayed gel coat has completed its heat cycle. The easy explanation is they have attempted to use a short cut in the application.
Many repair pros try to spray gel coat through a Preval® sprayer, since it is easy and convenient. This sprayer is an aerosol set up that allows mixing the ingredients in a glass jar. It is then atomized at low pressure using an aerosol source.
The low pressure requires the gel goat to be thinned excessively for spraying. This results in creating a recipe outside of the manufacturer’s requirements for optimum performance. Technicians have reported to us they try to use acetone as a thinner and create a 50% mixture. Every gel coat chemist says “do not thin gel goat more than 10-15% and use styrene only”

Gel coat is made for production factories “ready to spray” at 60-80 psi at the gun. Catalyzing at the gun tip yields optimum chemistry in this setting. The mold keeps oxygen away from the surface and gives the maximum hardness to the sprayed gel coat surface. When cured gel coat is pulled from the mold’s surface, there is absolutely no tackiness.

To duplicate this result in a repair application and achieve a tack free finish, you must:
-Spray pure gel coat at the recommended pressure (50-60psi). This requires an air compressor and proper spray gun or air brush.
-Spray it as close to the original manufacturer’s formula as possible (thin no more the 10-15%)
-Catalyze to between 1 to 3% with 90% MEKP
-Thin only with styrene.
-Most importantly, seal the surface at “peak exothermic cycle” with PVA.

All materials and tools are available from us. Using the proper materials and equipment will yield a more professional result, and a happier customer.

For more information on proper application of gel coat go to:

Click to access GELCOAT%20REPAIR%20GUIDELINE%20NCINSTGCGD-v%202.pdf

Ken Wolfe
Consulting Chemical Engineer
Multi-Tech Products Corp.