I've been told that it's not wise to mix resins when repairing fiberglass...i.e. if you have a fiberglass boat constructed with polyester resin, you want to use polyester resin - not epoxy - to repair it. What's the general rule of thumb here?

Jake,
I've always been told that epoxy will stick to polyester/vinylester. BUT poly/vinyl will not stick well to epoxy.

Rob is correct. Epoxy will stick fine to polyester and is stronger. But polyester will not stick to epoxy. There is no problem using epoxy to repair a polyester boat except that you will not be able to finish the repair with gelcoat (polyester)...Dan

Hi Jake,

A couple years ago, my boat was rolled into the side of the barn where I store it for the winter. A large nail was pushed through the side of the hull.

I repaired the hole using West Sytem products and finished with gelcoat (polyester) provided by Performance. The repair is completely invisible and strong. Below is an excerpt from what I found on the West Systems web site (search on gelcoat).

" Many of the questions concerning the repair of fiberglass boats seem to revolve around the techniques used when applying polyester gelcoat to cover a repair made with WEST SYSTEM® epoxy. There are several steps to a successful repair. It is important that these steps are done in the proper order to assure a well matched repair color.

1) One of the steps that is frequently left out is to apply a sealer coat of epoxy to the repair area. This is necessary to fill any porosity in the patch. Apply two or three very thin coats of epoxy, extending each coat slightly beyond the previous one. Warm the area with a heat lamp to speed the cure and to help the epoxy flow out nicely.

2) When the sealer coat has cured, wash thoroughly with water and a 3M Scotch-brite pad to remove any amine blush. Any blush left on the surface may inhibit the cure of the gelcoat. This very important step is often forgotten, and the result is usually an unsatisfactory repair.

3) Sand the epoxy coated repair area with progressively finer grits of sandpaper. Finish with 220 wet or dry.

4) De-wax an area twice as large as the diameter of the repair. Apply tape around the perimeter of the de-waxed area. Use masking paper to protect the boat from overspray. If there is a molded bodyline or corner near the repair, you may want to extend the color patch to that point. The same is true for a painted or vinyl stripe.

5) Sand the additional area out to the tape line with 320-grit paper. This will be the total area to be gelcoated.

6) Determine the gelcoat batch size for the size of the repair--approximately 80 square feet per gallon of gelcoat (20 sq. ft. per qt.).

7) Tint the batch of gelcoat to match the color of the boat. If the boat is fairly new and the manufacturer is still in business, you may be able to get gelcoat that will be a very close color match. If this is not an option, you will need to get the gelcoat from a FRP product supplier. Many marine distributors handle the gelcoats from various resin manufacturers. You will also need pigments to tint the base color to obtain a good match. These pigments are generally available from the gelcoat suppliers. Frequently, local repair yards that do fiberglass repairs will sell these products to the project user. "

Etc. (see Gelcoat Repair Techniques for the rest . . .

Thanks Kevin. I have an epoxy repair on my boat and gelcoat just would not cure on it. I'll try the proceedure you suggest...Dan

Dan,

The real key step is to be sure that you remove the amine blush. Sometimes folks try to sand the cured epoxy to remove it. I know from experience that it is a mistake. It just mixes it in. You need to wash the surface well, using a scrubby pad, as suggested by West.

Good luck.

You are right in that the by-product of the curing process of epoxy is the amine blush on the surface. Also another thing to keep in mind is that the amines will cause the bonds to fail between epoxy and anything else; gelcoat, polyester resin, bottom paint, fairing compound, etc. (even later coats of epoxy), if applied after the "green stage".