so the idea behind primer is its high build and easy to sand. So the high build gets some paint thickness up quick, filling all the low spots, and then being easy to sand lets you knock down the high spots. You wouldn’t sand to 1000 first, you’d clean it, sand to 320, clean it again, primer it THEN sand to 1000. Getting really technical you’d even want different sanding techniques for scuffing the foil for pant and then the final fair. Usually (if it were like the centerboard on a race boat) you’d use a 2 part marine epoxy primer. I would not - under any circumstances - rattle can a foil with rustoleum primer. The rustoleum has tons of wierd and angry chemistry that helps it stick to metal and deal with rust. You don’t want that chemistry on your foil.
If your foil is old and beat up sell it to some goober for half price and buy a new one, if you paint it (even a professional with top quality marine paints) you would only WISH you could get half of MSRP when you inevitably sell it.
Unless you are a professional sander with great technique and a proper sanding setup you will 100% do more harm than good. I have access to such a setup (festool finish sander and vacuum), deal with fancy marine paints on the regular, and i still wouldn’t be caught dead doing this to my foil myself.
For a board - esp a black board in the summer - this is fine because it helps a problem (heat) and while it slows the board down(100% doing more harm than good) its not dramatic. for the foil you’d be solving nothing.
Also, even sanding the foil is going to have consequences. Sanding the foil (for paint adherance then to knock down the high spots) is going to expose some carbon previously covered by factory paint - this will make the setup more electricaly reactive and make your hardware more suceptible to electrolytic corrosion.