I have heard 7 to 10 mils which is .007 to.010 inches is this correct? I does not seem like much to work with But I know that too thick will prone to cracking.

I had air pockets under the gelcoat in the same location on my transom. I picked them all open, filled, sanded, and sprayed gelcoat. I don't know how thick but I made several passed. It builds up pretty quick. Then sanded 300, 400, 600, 1000. Looks great.