lots little tiny blisters???
When you talk about lots of very small closely spaced blisters, that is usually a gelcoat adhesion problem. Often you can just peel off the gelcoat with a sharp chisel without getting into the laminate because there is often such poor adhesion in these area. Once the gelcoat has been removed,you will need to sand the surface, fair it, wash it down to remove any contaminants and then build up enough layers of a barrier coat (either epoxy or vinylester) to protect the hull from further blistering.
WEST system epoxy has a good monograph on blister repair. I''d take a look at that if I were you.
There is no way that I would use a belt sander as they tend to create a series of flattened spots and belts are very expensive compared to disks. This will project eat up alot of sandpaper. You can probably buy a high quality disk grinder for the savings in sandpaper costs.