Gelcoat question

[Alan]

The gelcoat on the transom of my 1980 DSII has many small pits (hundreds, maybe even a few thousand) that look like they might go all the way through the gelcoat to the laminate underneath.

I could see sanding down as far as necessary, but then what? Can new gelcoat be applied on top of a very thin remaining layer of old gelcoat? Or do I need to sand all of the old gelcoat off?

I've thought about painting the transom, but there's probably wax in the pits, and because they're very small, I suspect that even a thorough acetone wash wouldn't remove it.

[GreenLake]

If you have your heart set on gelcoat, you can definitely sand some (or all of it) off and apply new gelcoat. The problem is that gelcoat is really tricky to get to cure when it's not inside a female mold. It needs the mold to get a nice smooth surface, and it needs the mold to exclude the oxygen so it will not remain tacky forever.

Wax paper or acetate film can be used to simulate a mold, and there are formulations of gel coat that will harden in air (they have wax in them). Attempts to get gel coat to harden by using liquid release agent as barrier may work for some people - they have failed reliably for me.

The wax paper method has worked for me really well only when repairing smaller areas (where there's a "rim" of existing gelcoat that defines the surface).

Gelcoat forms a rather thick layer with a single application. You need to make sure you don't apply it too liberally or it may show stress cracks because it's more brittle than laminate. You wouldn't need to sand off all of the gelcoat, only enough of the surface to get to the good stuff.

Getting a good color match is tricky, so doing an entire panel (like the transom) is the way to go. (But perhaps do it in sections?).

As gelcoat is so thick, it's usually OK to sand and polish the top layer when you've applied a repair. Wet sanding to about 1500 grit more or less, then rubbing compound and finally something like 3M's Finess-it II Glaze.

Even if you sand off the gelcoat, you do need to remove any wax first, or you will merely smear it into the grooves created by sanding. Acetone is probably not the best to use for de-waxing. MEK ? or the use of special fiberglass washes for prep purposes might be better.

Alternatively to gelcoat you could apply an epoxy barrier. Either use an epoxy based paint or barrier coat for that purpose or apply a layer of laminating epoxy. That should stick well to the gelcoat and seal your tiny holes. However, you then must paint the epoxy as it cannot be exposed directly to UV.

If you dry-sail your boat, the range of solutions and paints is greater than if you keep it in the water all season. In the former case, you could use a PU paint with a catalyst as a top layer or any "top sides" paint for a boat. In the latter case you need to use something else below the waterline - but then, perhaps you may need antifouling there anyway.

If you've kept your boat in the water in the condition you describe you may want to first dry her transom out after sanding it - so you don't trap any moisture.

[Alan]

Thanks, GreenLake. I think you just talked me out of opening a can of worms.

The boat is dry sailed. It sat on a trailer in the previous owner's garage for at least three years, and I haven't managed to sail nearly as much as I'd like to. There are some ripples in the gelcoat on the transom, but they're very small and they don't deflect under pressure, so I'm hoping they're not a sign of delamination.

I've been looking for an excuse to paint the boat just to change the color, so this may be it. It's good to know that polyurethane is OK, since there seem to be lots of choices of brands and colors.

Lots of good alternatives here. Since our rainy season is just starting, they'll give me something to think about for the next few months.

[GreenLake]

Yeah, make sure your transom is really sound. Some DSs have rotten plywood issues as the transom isn't solid laminate. But if it's fine, then there's no reason not to paint it.

Just for completeness, I've read a ringing endorsement and report of someone's experience using Awlgrip - which is a bit expensive, but the stuff to go to when you want mirror smooth coatings. Can't remember where I read the report - Good Old Boat magazine perhaps?

If you paint the whole boat, do yourself a favor and make sure the hull, esp. the forward third, doesn't get brush marks etc. on it. Bumps there will slow your boat down. But you knew that already.

A friend did a Dinghy using SystemThree Linear LPU (one of the catalyzed PU paints) and it came out great with rolling on and then "tipping".