[rowinghelmet]

I once stripped bottom paint from a boat with chemical stripper and sanding. It's a pretty labor intensive and miserable job.

I would recommend sodablasting or walnut blasting. It's like sand blasting, but won't damage the gelcoat.

Fyi, the bottom will be dull when you're finishing. It is usually sanded with course grid sandpaper before the bottom paint is applied. You would need to re-gelcoat or sand and buff the bottom to restore the shine. This is also really labor intensive.

You might just want to re-paint with a white bottom paint like Petit Vivid White.