First step is to really determine what fabric you’re dealing with and the nature of the separation of the gel coat from the layup. Find out what kind of resin was used in the layup if you can. I would probably call a manufacturer of resins etc. and see if they can tell you what is compatible with polyester gel coat. Also ask about what to use as a filler if there is a void that needs filling. In any case, you can protect the fabric by taping over it while tapering the edges of the hole. I use black plastic electricians tape to protect all surrounding areas that are outside the damage zone.
Masking tape sometimes doesn’t stick well and it tears up so easily.
When you are satisfied with the filling and it has cured, clean it thoroughly with acetone. Wipe the wound thoroughly with acetone again just before applying gel coat. Brush the gel coat on as others have said–a bit higher than the undamaged surface. It will shrink a little.; if it does–no big deal. Just repeat the application process after the original application has cured. Keep in mind that you do need a working temperature of at least 65 to 70 degrees F to do any of this work… I usually let it cure over-night. .
You do not need waxed gel coat, or mylar , etc. to cover the curing gel coat, just cover it with waxed paper. Here again, I use black plastic electricians tape to seal down the waxed paper, because it sticks better and is easier to peel off.
When the gel coat is fully cured tape off all of the surrounding area to protect the undamaged get coat from being scratched while you are sanding down the new gel coat. Start with dry 400 to 600 paper, just to work it down closer to the surrounding surfaces… This initial sanding is very easy and requires very little pressure… When it looks like you are pretty smooth, switch to wet sanding with 600, then 800, 1000, 1500 and 2000. This all goes very quickly and it will look like you are done. You’re not. Now you will need a real good random orbit polisher.with a sponge type polishing pad. Use any cheap polishing compound and polish to suit. Wipe on your favorite wax, or other finish and you’ve got a brand new boat.
Actually in the finishing process, I remove all of the protective tape around the repair and blend the polishing with the surrounding area. If all of this sounds hard, or tedious–it isn’t and it is very rewarding.
Another little note is don’t try to fix too many damaged areas at the same time, or at least not any multiple larger repairs. You will only have about 20 minutes of working time with any one mixture (gel and hardener), so keep the amount of mixture small. Be sure you understand the right amount of harden per amount of gel coat. Check the instructions on the gel coat container. I generally use about one drop of hardener per teaspoon of gel.
You will find that with a little experience, small repairs can be made without all the protective taping as you will quickly learn to keep your sanding within the linits of the wound.