Sanding it all down is what some do(and advised), but that was too much work for me. My decks non skid is 50+ years but I left the non skid alone. It's been painted at least 3 times from the colors I sanded through. For perfection, I would have had to sand it out, it has some dings and gouges, but so does the rest of the boat. I painted on some medium aggression paint remover, rinsed. Sanded the smooth margins to get some tooth for the paint, then washed with TSP. No primer, I went right to finish coats.

I figured that I may just add some grit and put another coat on the non skid if it was slick. But after 2 coats(with a one part acrylic urethane), I thought it felt like it had enough tooth. I'm glad I didn't add grit, the pattern even after 50 years is still working, it's a pretty deep diamond pattern.