[SVCarolena]

Use a dewaxer (any of those discussed, we use Interlux) before you start sanding. Otherwise, you risk pushing the mold waxes and any other grease and grime into the fiberglass. You do want to dewax anything you are going to paint first, particularly the bilge. The boat mold had a release wax in it when the boat was built and you need to get that off the surface.

 

[SVCarolena]

I missed that part - many boats, including ours, do not have gelcoat in the bilge. If it has gel and paint, then you are only dewaxing that surface. Acetone would work, but I don't like working with the stuff unless I need to - very flamable, will do a real number on your hands, and I have yet to find rubber gloves that it won't destroy in short order. But it would work, as would all the other options that have been mentioned.