I recently polished up a 20 year old high top (also did my sailboat). You're probably going to need a heavy cutting compound for the first 1 or 2 passes. 3M makes a popular one - there are many others. I also found that polishing by hand is just not enough power. I bought a used rotary buffer/polisher and got pretty good results. Even then, I don't think completely new looking gelcoat is possible. But, you can get pretty close.