From what I read and research, the two best choices for non-skid are Interlux Interdeck and Kiwi-grip.
Interdeck has rounded non-skid aggregate already mixed in, so it's not as sharp as using, say, sand or pumice. Much easier on skin. It only comes in a few colors, grey, blue, tan, white.
Kiwi-grip is a pudding-like consistency material which is rolled on. You can control the aggressiveness of the finish by what type of foam roller you use to roll it on.
I've seen reports of many people using either product and being very happy with the results.
Others report painting non-skid areas with regular paint, and provided that the non-skid pattern was still pretty good, they say it stays non-skiddy after painting…
I think if I had to paint non-skid with Interdeck, I'd do a fair amount of sanding to get the non-skid areas fairly smooth, so that the Interdeck provides the grip. As for Kiwi-grip, I believe they recommend putting it right over non-skid. Because it's thicker to retain the texture of the roller, it should fill in any existing non-skid pattern.
