Tag Archive: gelcoat

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


I would like to add a slip-resistant surface to the bottom of my bathtub.  How do I do it?

Multi-Tech Products Corporation offers components capable of providing a slip-resistant surface to bathtub bottoms, shower bases, tile or concrete floors.  The system was designed for applications where a patch area or a defined non-slip surface area is required to avoiding the entire surface.  Obviously, no surface treatment can totally prevent falls.  These products will increase friction and reduce the likelihood of a fall.  The components include either a clear coat or a color-matched coating with a semi-transparent powder added to provide a textured surface.  The Quick Glaze system provides a great textured surface for porcelain, cast iron, gelcoat and acrylic bath products as well as tile, concrete or metal surfaces.  If the procedure is followed, the final result will have great adhesion to the existing surface, and the appearance will be a consistent textured surface as shown.


Yes, cracks in spas or bathtubs can be fixed.  First, let us understand what a crack is.  Spas are subject to both cracks and crazing.  A crack is a well-defined, easily seen separation of the surface.  It can be a single crack or have a few branches.  Whereas, crazing consists of hundreds of small, often difficult to see without magnification, cracks covering a large area.  Cracks and crazing have different causes, and crazing is more difficult to repair.

CRACK                                                                CRAZING

Cracks can occur due to mechanical impact, or due to excessive stress in the surface, which becomes higher than the material strength. Multiple causes of excessive stress exist. These are exaggerated when the surface gets hot due to sunlight, etc. There can be multiple cracks in a surface, and they go through the entire surface layer.  Crazing is generally caused from chemical attack.  The chemical aatack can be from the visible surface side, or the underside.  Crazing from the underside has been predominately caused by excess styrene in the fiberglass reinforcement.  Numerous chemicals have been known to cause cracks and crazing from the visible surface.  These are normally caused by the owner.

Cracks in acrylic or gelcoat spas, bathtubs or showers can be repaired using Multi-Tech Products Corporation (MTP) surface repair kits and procedures.  Visit http://www.multitechproducts.com to order these materials, and to learn how to use them for repair.

Many bathrooms contain a combination, one piece shower and tub.  These are generally made from fiberglass reinforced plastic (FRP) coated with a gelcoat resin.  They can develop cracks in the tub bottom caused by inadequate support to withstand the weight of the users.  Repair of these cracks require adding reinforcement to the underlying support structure.  MTP also has products and procedures for this operation.  Please go to the website and find the procedure for tub bottom replacement.

Go to http://www.multitechproducts.com/pages/procedures.html for more information on repair procedures.