Restoration of fiberglass hull

Hello everyone,

At long last, I’m getting around to returning a used US12 I acquired some time back to the local waters. But I need to correct some issues that I have with it first. The main one is what looks like stress cracks around the hull. They appear to be in the gelcoat. Do I sand and fill the outside? Is anything needed inside also?
The other is repainting the hull. I have only painted a few polycarbonate RC car bodies which is entirely different. Do I sand it down to the glass first, or just rough sand, primer, and so on?

Thanks in advance

If mine, I would start with a sanding to roughen the surface. Depending on how big/deep your cracks are, it is possible that spraying a “High-Build” primer might fill them. If they are deep, use some body filler (Not the big can of “BONDO”) - but the small tube of red or green surface filler usually used for pinhole filling. Once sanded, wipe entire hull with a clean cloth and denatured alcohol. Then apply multiple coats of thin sprayed primer. Sand lightly and then paint, also using multiple coats of a light spray - and be sure the primer and final paint are compatible, otherwise you will wind up with orange peel, bubbles, or peeling paint. The light coats of paint will eliminate too much thinner or solvents which can also cause finish problems. Light paint spray, let dry - light paint spray, let dry. etc.

Totally agree with Dick’s advice - only thing I would add is that the gelcoat stress cracks should also be investigated inside the hull. If there’s any sign of the cracks running through the laminate (they’ll show as white lines a bit like dry laminate) then they either need to be sanded (40 - 60 grit) or very lightly ground out and then replaced with a new fibre glass / resin mix. If multiple layers are needed to replace the original thickness start with a relatively small piece of cloth that covers the repair area with a small overlap all the way round and then add sufficient subsequent layers, each one with a slightly larger overlap until required thickness is achieved.

Owing to the relatively thin laminate of our hulls the flexibility of the glass/resin matrix is far greater than the gelcoat and so you can be fairly certain that the laminate is ok but it should definitely be checked - it’d be incredibly frustrating to concentrate on gelcoat repairs only for them to open up again a while later.

Best of luck with it,



Thanks for the replies. Will look into some of that filler. Would also like to add an additional layer of glass on the inside, but think I may have to remove the lead to do so.
Attached are a couple of photos of the area in question. The inner photo is highlighted by a back-light which shows the extent of where the crack lies. There doesn’t seem to be a white line on the glass without the lighting though.

Again, thank you. It would be a shame to not get this hull back on the water.

Hi ,
IMO before starting any external repair or adding internal laminations, I would suggest first to check if the hull is leaky somewhere not only where the cracks are visibles.
You may discover other points of interest.
To do that it is sufficient to put water inside up to the water line and wait to see any leakage indication (bubbles) once done with the hull right up, you tilt the hull by some 35°/45° on one side and on the other and verify for leaks.