MN3-Funny my son and I talked about that it would make a great potato cannon.
Pete yes I've used acetone. Enough to get 10 drops from the mixing stick. Not too scientific. If the gel coat is really old and thick buy new it is fairly cheap. I'd say about the same thinning I've done with Lacquer paint if that helps. Once thinned you may have to do multiple coats with a little flash time in between.