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Avoid the grinder, it make too many low spots that you will have to fix before painting. I used an air sander with 40 grit on my son's 13' that we sanded all the cracking jell coat off of. I used west epoxy on the parts that were down to the foam for filler, and 3M marine body filler for smoothing the rest out. For the holes that were mentioned earlier, a couple tubes of Bondo glazing compound fills them nicely and is really easy to sand. We even filled the recessed floor seams with it to smooth it out. I just painted another coat of Interlux Poly on his interior today. We did two coats of Interlux two part epoxy primer on the bottom and top of the hull. The results are amazing. For gel coat spray they do make a special gun for that.