I had a bunch of stains on my cabin top around the grab rails from previous owners who chose not to mask the area when varnishing. My method was a combination of a couple items here, started with light sanding with 1000 grit wet, removing as much as possible from the top level of the non-skid. After that, I used a single edge razor blade(several of them) to scrape in between the grooves and nooks and crannies. It took me a while with a lot of patience, but I got just about all of it off, with no visible change to the non-skid. For the smooth surfaces, I always find a single edge blade works great, just be careful not to scratch your topcoat.