Have you tried an orbital sander with very fine paper and then rubbing compound? It does seem to work as long as the gel coat is thick enough and if it doesn't work you have a nice surface to paint on to!
if you are painting because you cant get the jel coat clean it is proberble that you are using the wrong tools and cutting compound.
You need to use a high speed buffer capable of about 2000 revs (auto buffers are the best) and a cutting compound such as Farecla G3 liquid compound applied with plenty of sprayed on water a hand garden spray works best. the reason for the high speed is because jel coat is a extreamly hard compound.Also jel coat is (depending on the boat) not that thick, about 0.020in so care must be taken not to use this treatment every year, normaly you can see when you are getting to thin, it starts to look a bit grey.....but if you are contemplating painting its worth giving it a go
Have already tried that and the finish is very patchy, I've tried quite a few cutting compounds, including the 3m one which I have to say was excellent.
However the finish seems to be free from oxidisation and the colours arn't deep and are patchy.
Used TOPLAC for a cove line, not too good though, brush marks every where but then I'm not a painter. It was also a soft paint cf. the 2 pot polyurines. I will be repainting this year and i am tempted to try Brightside, I guess its too new to have much of a following.
I've just buffed up my topsides with G3 and a polishing machine (automotive) using a foam mop. The result, 16 year old topsides you could shave with. Worked a treat with the coachroof too, though the mop got a bit chewed up on the fittings!
£ 23.75 + VAT for the weekend from HSS. Don't buy the lambswool bonnet they'll offer you. Get a foam mop from a pro motor factors, say £ 4.00 or so. They will also have the G3 too.