Some simple answers to a simple project: First, the gel-coat does not need to be removed - only sanded. 400 grit, (wet) followed by 600 is sufficient. The "grey" you're seeing could be primer - if the finish is paint
- rather than gel-coat, but I doubt it. It's probably only the fiberglass/resin layup that was applied to the gel-coat in the mold. A quick test would be to wipe a small section with a cloth dampened with acetone. If it is paint
, the finish will come off. If it is gel-coat, it will have no effect, unless saturated for quite some time. Only sand the area/s to be painted. The type of paint to be used on gel-coat is different than on metal, but any good body and paint shop will know how to paint a gel-coat finish. The "spider" cracks are fractures in the surface of the gel-coat and can be caused by excessive exposure to heat, by impact with another object, or a combination due to too-thin application of gel-coat in the manufacturing process. The gel-coat is sprayed in the mold first, followed by layup of f'glass cloth and mat, or chopper-gun sprayup.
Hope this helps, noob. I'm new to this site and am having trouble navigating, so thought you may be too. I'm sure, when you find them, others will have more info than I have offered. But, it really is simple. I'm presently sanding my "Lil Tote" (73) in preparation for painting
Good Luck! Carl