I wouldn't bother. I've tried it both ways and the easiest (mind you I used RustOleum Marine
Gloss one part enamel) is to stay away from fittings with your roller and large foam brush, it is remarkably easy to paint around the fittings with a small artist brush. You don't even need to tape the fitting off, if you have a steady hand it will look perfect and if you are sloppy it is easy to get a little paint off the fitting with artificial steel
wool pads. It is impossible to see the blend line using an artist's brush.
Not to mention it is going to look really bad if you paint under and around the fittings until you go back and paint the deck. Also, be careful using a power sander on your nonskid, or sanding
the nonskid excessively unless you plan to take the entire nonskid pattern off. Even if you put several coats of sanded paint over areas of denuded non skid it will be quite apparent due to the difference in texture.
Attached is a photo
I found which illustrates this: the slick area is taped off and the nonskid area is being painted with sanded paint to make it nonskid again. After one coat of paint without sand nonskid gets slippery. The bazillion holes in the deck the PO's placed were visible until about the sixth coat.
Another trick is if you don't want your deck too shiny, just paint it when you think it will be sunny without checking a weather report. When it rains an hour after you paint it dulls the finish nicely.