Gelcoat Repair Method Advice-Pease Vote!

I will be attempting to repair significant portions of gelcoat damage to an old, oxidized, white Sundowner that was mistreated a bit. There are probably 6-10 areas of what I believe to be only stress cracks. The repairs aren’t really needed, but I just want to have some fun repairing (if you call repairing gel-coat fun). With so much to do, I have a concern doing it the easiest way possible, but would also like it to blend in in the end. I do realize I’ll probably have to remove the oxidation from the rest of the boat in order to get a good looking color match in the end.

Pls vote for a method below. I would especially appreciate any advantages or disadvantages to either method that you can think of.


Method #1: Regular Gelcoat Repair

The local canoe shop recommended using regular gelcoat colormatched to my white hull and wet-sanding to blend. He recommended using “______ balloons” (silica balloons, I think) from a marine store to thicken the gel so it wouldn’t run.

Method #2: Cliff Jacobson’s Method.

He basically recommends white polyester putty or gray auto body putty and then spray painting and wet-sanding to blend. Here is his procedure:

Mending gel-coat

. . . This procedure is easier and faster, and the finished repair is invisible.


•White polyester putty (available at marinas) or gray auto body putty. Use epoxy if you want a stronger repair. Thicken the epoxy with colloidal silica, as suggested in number 2 below.

•Sixty and 100 grit dry sandpaper, and 200 and 400 grit wet-dry finishing paper.

•Matching auto acrylic or epoxy paint.

•Fiberglass boat wax (it contains a mild abrasive) or paste wax and pumice.


  1. Pick out the shards of damaged gel coat.

  2. Catalyze the polyester putty (use extra MEKP for a hot mix) and work it into the break to overflowing. If you’re using epoxy, stir in colloidal silica until you get a peanut-butter-thick mix that won’t run.

  3. When the resin has cured, sand it level. Finish to silky smoothness with 400 grit wet sandpaper.

  4. Paint the patch. Later, use a mixture of paste wax and pumice to blend the paint to the hull.

I have both microballoons and
colloidal silica, either of which will thicken resins. I only use microballoons when I want a lighter, easier-to-sand result. Probably in your situation I would use colloidal silica for hardness. But it will make little difference. West supplies either thickener in cans, available at many boat shops. I can’t speak to the need to thicken gelcoat material, as I work only with West epoxies. Someone else will know.


– Last Updated: Dec-04-09 5:54 AM EST –

do either method

micro-balloons will thicken something that doesn't need thickening and will make the end result a dull spot.

micro-balloons are what gives the dull finish on some kayaks for deck seam coat....duller than normal gel finish

and puddy...well

I would rather just use unwaxed gel and spray with PVA to let cure...then sand and buff (my personal preferance)

Best Wishes

gel coat
1) mix small 1-2 oz batch… apply with throw away brush… let cure until you can touch it and leave a fingerprint… apply next coat of 1-2 oz batch… repeat for third coat until it is thick enough to sand down…

2) on last coat apply mold release (I used a throw away brush with a very wet tip… all it has to do is cover the gel, so don’t brush it in) let fully cure and sand.

Mold Release?
What is the mold release for?

Also, it sounds like you do not use any thickener, either. Will it be hard to keep the gelcoat from running? Or is that your point-to use many light coats?

Thickening . . .
So you would suggest that it’s really not necessary to thicken the gelcoat?

Also, could you tell me more about the unwaxed gel and spraying with PVA as an option? Do you think that would be better than the putty option or is it just, like you said, personal preference?

I’m definitely temped to do the putty or some other easier method due to the amount of cracked gelcoat that I have rather than dealing with a large amount of runny gel-coat applied in several layers.

I understand what you are saying about the microballoons making the finish dull however, in your opinion, do you think the finish would still be better than putty and wet sanded acrylic paint in the end? The microballons would make the gelcoat easier to work with, right? I may be able to accept a dull finish in the interest of actually having the time and attention to complete the project. With brown resin already covering all of the cracked spots (which I have to remove), it’s already a pretty ugly boat!

(Anyone else feel free to chime in as well)

that’s right
3 light coats… I tried with the plastic wrap but that caused air bubbles… then you have to sand between coats…

the mold release was sold right next to the gel coat at Boater’s World… it washes off with water… it will run like crazy but don’t worry about it beause it washes right off…

Get 4 mixing cups, 4 throw away brushes, gel coat, mold release…

I did a lot of research on this and my repair worked perfectly…

Dip the brush about 1 inch, flick off the excess, don’t wipe the brush on the edge, then apply it nice and thick, but not too thick, don’t brush it in…

Use the exact amount of hardner… it will be ready for the next coat in like 10-15 minutes (follow directions above)

If you can spray the mold release that’s better, but I ended up brushing it on that worked fine. (use a dripping wet brush and just dab it on as carfuly as possible right when the gel starts to cure)

use 60, 80, 120, 240, 400, 600 grit sand paper…

you can also use polishing compound at the end…

the gel coat should be thicker than what you need to sand down…

this only took a few hours from beginning to end…

There’s no need for mold release…
…if you buy “finish” gelcoat instead of the more common “laminating” gelcoat.

Thickening isn’t necessary
First off, buy “finish” gelcoat, as it’s designed to cure in the presence of air, which typical “laminating” gelcoat isn’t. That eliminates the need to use a Mylar sheet or PVA mold release to cover it.

With liquid gelcoat, you can build it up in layers if need be. If you have areas where you need to build it up quickly to a substantial thickness or where it will be difficult to keep it from running, you can get finish gelcoat paste in cans (West Marine carries it). It’s thick and will stay where you put it, but it doesn’t self-level, so it will require a bit more sanding.

I have instructions on repairing gelcoat at:

I’ll stick with
the laminating gel coat, thank you.

Suit yourself…
…but perhaps there are other people who don’t want to deal with the extra work involved for no gain.

is really no extra work involved.

don’t need to do layers , gel can be put on in the same way as waxed gel…when all done and ready to walk away…I have PVA in a cheap air brush(it can also be dripped on or lightly brushed on, if you don’t have a sprayer)…just give the area a light spray.

PVA is water soulable and just washes off as You wet sand…no wax in the mix…none needed

You might want to give it a try.

Working time is somewhat more user friendly and before you spray the PVA, if you see some place that needs a little more it’s not an issue because there is no wax floating it’s way to the surface during cure. I also think that unwaxed is friendlier to spray. but I’ve always been a little concerned with the spraying of waxed so this is an untried belief on my part since I always use unwaxed.

Best Wishes


I have used PVA…
…and I have a bottle of it at home. What I’ve found is that it doesn’t always work. With finish gelcoat, I never have that issue, there is less mess (no PVA dripping on the floor) and I don’t need extra brushes or a sprayer. It’s just plain simpler.

If you prefer to use PVA, fine. I don’t.

Another vote for Door #3
No thickener necessary.

PVA isn’t that hard to use, and works much better than the pre-mixed “easy” stuff (in my experience).

Practice to apply a gel coat that’s juuuuuuuuuuust thick enough that it doesn’t run. Sand between coats if you have the time and patience.

If you didn’t have the time and patience, you wouldn’t be making cosmetic repairs.

It’s fun winter stuff. Enjoy.

“Works much better”? How so?

– Last Updated: Dec-05-09 8:14 PM EST –

I've used both and I haven't found any difference other than that I don't need to screw around with PVA when using finish gelcoat. The finish quality and durability seem to be identical. If you know of any tests that show any performance differences, I'd love to know about them.

Summerpaddler … Please note
The cracks you are attempting to touch up probably got there from overloading or a weakend laminate. Merely filling them up without addressing why they happend might be an exercise of frustration as they WILL crack once again once loaded a time or two. Take a look to see if any of these areas need beefing up or re-do of repair.

good advice
Was wondering if someone was going to bring that up.

Stress cracks come from the composite structure moving under a very brittle gelcoat that won’t allow much movement without cracking, specially as it ages. Can happen from impacts with something which doesn’t mean the structure is really damaged in any way (though it could have been) and/or can happen because the composite structure was weak to start with and flexed more than the gelcoat would allow.

Either way, the cause of the cracking needs to be adressed first and any damage or weakness repaired before you waste your time playing with cosmetics that will indeed return as has been mentioned.

Bill H.