Messing with Marine Tex

Maerin said:

Bondo or other polyester or vinylester based filler/fairing compounds work really well for overhead filling. If you need to thicken any kind of resin or epoxy, use silica or fumed silica. It's inexpensive, and can thicken anything from resin to epoxy to gelcoat. An easy filler for gelcoat dings is gelcoat thickened with fumed silica, add a few drops of wax to make the surface cure hard, then just squeegee the mix onto the prepped surface. The most important step is to make sure the hardener is the correct proportion. Most accurate method is to use a gram scale and weigh it to get the proportion correct. If you're filling holes in a gelcoated surface, the thickened gelcoat will fill the void and provide a sealed surface in one step.

Gelcoat and poly/vinylester materials as well as the hardener have a shelf life, so it's difficult to keep those products on hand for a long time for small repairs. Epoxy has a very long shelf life.

Be careful in using epoxy as a filler or fairing if you plan to gelcoat the repair, as the gelcoat (polyester or vinylester) won't chemically bond to the epoxy and can detach. Not so the other way around, epoxy will bond to poly or vinylester resin without issue. If you use epoxy fairing, the repair should be painted to match surrounding gelcoat.

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Easting said:

Very good information Steve. Thank you. Please explain the wax. “add a few drops of wax to make the surface cure hard”

What kind of wax? Sorry for a dumb question.

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Maerin said:

Not a dumb question at all. Gelcoat won't cure out to its typical shiny hard surface unless it cures in the absence of air. Think of gelcoat sprayed into a mold during construction, it's the 1st layer, then subsequent layers are applied as the hull is fabricated in reverse- from the outside in. The gelcoat surface is designed to stay "active" so subsequent layers chemically bond. The mold is sprayed with a release compound that prevents it from adhering to the mold. With repairs, the gelcoat remains "tacky" unless a mold release is sprayed onto the surface to keep air off. Or, a small amount of wax is mixed into the gelcoat resin to accomplish the same thing. So if you purchase gelcoat, you'll see it "waxed" or not, indicating that the wax is already incorporated into the uncured gelcoat mix, so that gelcoat can be used to spray onto a repair and it will cure out hard so it can be sanded/buffed smooth. If unwaxed gelcoat is used, it will remain tacky, and it can't be buffed. Alternatively, wax can be added (drops) to the gelcoat as it's mixed for application thus reducing the need to keep multiple cans of gelcoat on hand.

Some repair shops also use a hardening agent for repairs, "Patch-Aid", "Duratec", or other brands that thin and harden gelcoat for spraying. Some of these products include wax in some form to promote a hard surface cure. The problem with gelcoat repair is that it's difficult to mix small batches with enough accuracy to insure the gelcoat cures properly and won't yellow or otherwise deteriorate in time that it stands out from the adjacent area.

I've done enough gelcoat spraying on my own boat to know that it's a bit of an art form, particularly when blending to an existing surface! Even an experienced pro can complete a repair that yellows out and looks bad after a year or two, it's simply the nature of the media and trying to match differing formulas. If the repair is over a large enough surface that can be easily delineated from the unrepaired area, it's much easier to spray polyurethane paint, and a better success rate with color match. My gray gelcoat is a real bugger to get to match, since the color can vary slightly in different areas of the hull!

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Rufus said:

Says no need for wax. Do you suppose it's already in the can

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