The power washer is the best i've found. The problem is that it will stay clean for maybe a week until the dirt/mold returns. The basic issue is that the gel coat is highly oxidized and porous. You can clean it all you want but if you can't seal it or polish it.... your screwed. It will be constantly dirty.
Like I said, I tried paint but because you can't get the pits of the molded non-skid clean or sanded, the paint doesn't stick well. Plus, it looks bad as the paint pools into some of the low spots but not all of them so it makes the non-skid surface look irregular and sloppy.
Hopefully someone will chime in with some helpful advice?