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.
Depending on how bad it is (pictures?) sanding first.
1.000 Grit
2.000 Grit
Buffing
And then 3Ms Perfect it 3-step polish.
Don’t use a regular buffer, go for a forced random orbit like this one at Harbor Freight. Have mine for 3 years, polished 5-6 cars + my Van from the base up.
Linear Mode
