I would start at 800 and work your way up. If you haven't done anything like this in the past take a look at some of the Shurhold videos. They have a pretty basic explanation and their buff magic is a pretty good product but a little pricey. You can use a DA like they used in the videos on the exterior, but I wouldn't recommend it for the interior until you have a good handle on the process. An outside radius should be done by hand or you will burn through the gelcoat quickly.Wet sanding, that's about the only thing I haven't tried. Any idea what grit paper they use? I've bought so many products and different pads for the buffer but I just can't make it look pretty.