Yes Styrene works better than Acetone. However the stuff is pretty caustic. I'm not an expert on the subject, but according to another woodbe expert who I trust on the subject says that MEK is better for thinning than Styrene or Acetone. Like anything, everyone has an opinion.
My 2 cents.

When Gelcoat is sprayed on at the factory it is 20,000 of an inch thick, any more than this and it will "Bog your boat down". Before you sprayed your gelcoat on did you happen to wipe the area in question down with any thing.Because the grey stuff your going over is more than likely some bottom antifouling paint and if you used some sort of reducer to clean it, you could of inadvertantly activated this coating. Bob G.

I did use acetone to clean the hulls after I finished sanding. One of the big reasons that I decided to take all of the bottom paint off was when I wiped acetone on it, it caused the paint to break down some making the smooth area very rough.

After reading everyone's post I took off the gel coat that never set up, resanded the area, and reapplied a new batch of gel coat. This time I put a couple of heat lamps over the repairs making sure that the area was warm enough for everything to "kick". (It's still 50's here in Virginia) Last night I took a look at my new repair and found it to be hard as a rock.

Thanks for all of the suggestions, and all of the valuable information.

Alec

MEK is methy ethyl ketone, which has one more carbon in the chain than acetone. So its solvent character is the same, just less volatile and slower to "Flash Off". Styrene is the monomer from which polystyrene is made, so when mixed in as a thinner it will become part of the polymer. Caustic? I don't get that part: There is already a lot of styrene monomer in the unpolymerized resin in the can. So you don't want to huff it or breathe it unnecessarily.

I'd be careful about MEK.

It melts plastic and strips concrete of anything on it.

Some powerful stuff.