The reason that wet sanding is necessary is that gel coat, unlike paint. is somewhat porous. The oxidation is easily removed by wet sanding (the key word being WET!) but if only using compound you will just be pushing all that oxidation down into the pores. This results in a blotchy look in 6 to...
That appears to be an old repair that didn't hold up too well. The blisters are probably due to some bondo type crap that was used to fair the repair. As Scott said - sooner rather than later, it's most likely leaking.
Most, but not all, epoxies produce an amine blush on cure. This amine blush is removed with water. Acetone and alcohol only smear the blush, water is the only solvent for amines and should be scrubbed with a scrub pad and lots of water. Just yesterday I made the mistake of assuming that a heavy...
