Alec,
the first thing I would do is use acetone to remove all the gelcoat that is not hardened. You may find that the whole gelcoat repair comes off with acetone. The areas that apear had may not be fully cured. If they aren't, on hot days the gelocoat will stick to whatever it comes in contact with, ie hull cradles or sand even. This is bad. I have had this happen several times, usualy with old gelcoat. I would note that the shelf life of the stuff is limited and the can should never be allowed to freeze. IF you bought it from west marine or similar it may have been on the shelves too long.

for small patches it is easier to brush it on with a foam tiped brush. Get the correct build up by adding multiple layers. I usualy add 2-3 coats of gelcoat, waiting just long enough for it to partialy tack between coats (20-30 min if it is warm0. I have had mixed results thinning gelcoat with acetone. To spray it, I think I use styrene instead. Add pva or i it is a small spot, cover with wax paper or plastic rap. Anything that prevents oxygen from getting to the gelcoat will work.

Good Luck

Eric Anderson