Lots of good info here. I also use scotch brite to prep plastic, it goes faster if you use a gritty paste (forgot the name) along with the pad. Since you asked about when to wet sand. Wet sand after you've cleared and its dried. You can nib the little spots of trash with 1000-1500. Once you get the trash taken care of step out the whole thing with 1500, 2500, 3000. If you could get the 3000 for a DA sander, it buffs like butter once you finish. Good luck, post pics and the method used when you get it done.