I did just about the same process with my 1970 Mariner which had sat outside and uncovered in a boatyard for almost 10 years. It was rough to say the least.
The hull was pretty easy compared to the topside with all the nooks and crannies.
What you really want to do is start with the least aggressive method as you are removing gelcoat and there's only so much there.
Using a rotory buffer, [not a random orbital] try to polish a spot. If that doesn't work, try compound, saving sanding as a last resort.
I first tried wet sanding with an air powered DA sander using 1500. I found that as the topside, especially the long flat surfaces, were not perfectly smooth, I was seeing sanded high spots and un-sanded low spots. It was giving me a bit of a stippled appearing effect with lighter and darker spots. I also felt like I was removing too much material.
I then tried green Scotch-Bright pads cut into a circle and stuck to hook and loop DA pad. If you cut the Scotch-Bright a 1/2" or so larger than the pad, it helps with getting into tight spots.
I think the green pads are the equivalent to 800 grit, and the gray pads are about 1500, but they don't load up and aren't as aggressive as the same grit sandpaper.
I used the pads with a heavy solution of Simple Green and water.
It worked very well leaving the surface clean, smooth and ready for compound and was flexable enough to smooth the high and low spots while removing less gelcoat.
Regardless of method used, go very light on corners.
robj
