I restore old gelcoat quite differently, depending on the severity of the oxidation/degradation of the surface.
I use a pocket microscope and see how 'rough' the surface is initially. Then make a judgement of what grit to use (1000 grit ----> 2000 grit WET and Dry sandpaper) and wet sand the entire surface down to FLAT. The microscope will show when its FLAT, so I dont take off too much gel. W&D is 'lubricated' with detergent (a few drops per gallon of water) to keep the sandpaper clean. Sanding is done with rubber blocks. Wet sanding is VERY fast way to remove the oxidized layer.
IMHO if you 'blindly' compound away at gelcoat you can easily remove tooooo much of the surface and lessen the life in service of the gel (.... but gel is easy to re-spray).
BTW - before I start restoration I 'soak' the gelcoat with STRONG caustic detergents to remove any old 'dead' wax that may be in the 'pores' of the gel; old dead wax seems to accelerate the oxidation process.
Then when done and Im sure the surface is FLAT, I then use knobby foam rubber polishing discs on a variable speed auto body shop polisher using 1000-2000-3000 grit compound; actually I now use 3M Finese-it, then 3M Perfect-it 3000, then use Autobody shop 'glazing' compound, then Wax.
This is essentially how NEW boats are polished/buffed, etc. when they are pulled out of their molds.
