@jjkewll First, clean the hole thoroughly. Perhaps the best repair material is epoxy mixed with either chopped strand fiberglass or milled fibers. The epoxy will provide the strong bond and the fibers will give the structural reinforcement. To hide the repair, sand it flush, then either use a matching marine paint or pigment some epoxy to match the surrounding area. The surrounding area is probably gel coat, so it would seem logical to use gel coat for the surface.

@YANKEESRULEMLB For the final layer, use mat, not some course weave fabric. When it is all cured, clip off the wire, and grind the repaired area smooth to a level just lower than the surrounding gel coat. Wipe with acetone and apply gel coat. You can then either apply Mylar to the wet gel coat for a mirror finish, or allow it to cure then sand a buff.

@YANKEESRULEMLB It is difficult to give a precise answer without knowing the size of the damage. This will be a general answer. You will need to adjust it for your repair. First, grind out a ‘scarfed’ area as described in the video. If there is a hole in the fiberglass, it will need to be closed so that more fiberglass can be applied. If the hole is 2″ in diameter, cut a 3″ diameter circle out of cardboard.

i hate to say but 20 mm is 3/4 of an inch or if its referring to another measurement then maybe don’t use the term “mils” because that mean millimeters in metric countries (or milliliters when talking about fluids)

The purpose of grinding is more than just cleaning. The damaged area of fiberglass must be removed. The structural reinforcement (fiberglass fabric) of the repair should duplicate what was removed as closely as possible. A patch placed on top of a damaged area will not be as strong. If the repair is strictly cosmetic (not structural), then wiping with acetone is probably adequate, though certainly not a guarantee that all contamination has been removed.

Great how to video. Thanks for posting! Is the repair the same for automobile bodies? I have some ‘extra’ holes in my Corvette floor pans that I need to seal.

Yes, the same method, except you would work from the one side only instead of the back side and front side as shown in the video.

That depends on what the surface of your boat is. Some boat hulls are painted below the water line. Gel coat would not be appropriate there. If the surface is gel coat on a fiberglass hull, then gel coat could be used. Proper preparation is crucial. Also, realize that gel coat does not ‘self level’ like paint, so it will need to be sanded and buffed to create a smooth surface. That is challenging for large repairs.

Gel Coat will stick to most plastics, including Lexan, unless they are carefully prepared with a mold release. Two plastics that gel coat does not stick to are polyethylene, and poly propylene. If a mold is made from those plastics, no mold release is required.

The molds used to make boats are made out of fiberglass with a highly polished gel coat surface. They are carefully prepared with mold release. Otherwise the new gel coat would bond to the gel coat in the mold.

what is the name of the plastic you used for gel coat mold. . . Is this the only type of plastic that works?. . . or does most plastic sheeting work like lexan etc. . .

A properly done repair can be as strong as the original. However, areas of stress, or complex shape require a high level of expertise. The repair materials must match the lay-up schedule and structure of the original. This is often difficult for the novice. Repairs that have safety concerns should only be attempted by someone with experience and knowledge of fiberglass structure.

OK. . TY FOR SHARING