wfis11, I'll try that. Right now I'm testing Dawn, CLR, Tough Stuff, and Pinesol (separately). I've put a drop of each on a "dimple" in the non-skid floor. Letting it sit 15 mins. My hopes on that aren't high. I'll try your vinegar solution if these don't work. I've read elsewhere some people use muriatic acid. They say it won't hurt gel coat if you wash it off immediatey. You can't just wipe something on and rinse off those dimpled floors. Something has got to get down to the bottom of the dimple where the specks are. I have hundreds of them, maybe thousands.
First off, scuffing the gel coat will only make matters worse at it provides a better "foothold" for dirt. That will generate more mildew.
Here's what works for me:
1. Wash the entire area with a solution of Spray Nine mixed with bleach (1/2 cup of bleach per gallon) and let the wash water sit for at least 5 minutes before a thorough rinse. Keep fabrics away from any splash.
2. While the decks are wet, use Woody Wax, which is applied to a wet deck brush and work it into the porous surface and non-skid areas --using the instructions on the Woody Wax bottle for non-skid. Once it is dry, you blast it with a high pressure garden hose stream and then you towel dry everything.
Caution: Until you dry everything off, things will be slippery, including your non-skid areas.
If you are willing to put a little more effort in the short term to avoid more work in the long term, you might buff the smooth gel coat areas with a light compound and then follow up with a good wax, like Collinite 925, and only use the Woody Wax treatment on the non-skid.
If your gel coat is heavily oxidized, the compounding step is highly recommended for your smooth areas. If the deterioration of your gel coat is such that you can't buff it completely smooth, you could just treat everything as if it were non-skid and use the numbered process above.