I had the same problem on a previous boat. Tried paint thinner, acetone, et al, and nothing really worked well. I did try some fiberglass boat paint remover and it sort of worked, but it will soften the gelcoat if left on too long.What really worked for me was to stop trying to remove the Cetol, and instead covering it. I used some Interlux topside white with an artist's brush to just put a dab on the cetol spots. Bingo. No more cetol (visible, that is).