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The problem with sandblasting will be ensuring an even removal of the gelcoat to leave a flat surface you can re-coat. It's not like blasting paint off a car, where you have a relatively soft paint over relatively hard metal - both the gel and the underlying resin are about the same hardness, so an operator would need a great deal of skill to keep from making a hash of it.

You're better off getting a big sander and a bunch of 80 grit paper (maybe even 60) and just sanding down to the glass. Then I'd be inclined to go with a barrier coat to keep water out, and then something like Line-X bedliner.

Tim