The black finish is a "chem-resin" type finish designed to resist most all lab chemicals including solvents, so that's why the stripper didn't work. I would suggest starting with planing the chem-resin material off. My guess would be that the chem-resin material will just quickly gum up any sanding media.
"chem-resin" Danger Will Robertson, even I would not be snorting that dust.