Aerosol spray paints do not hold up

gel coat works, once you master it. I never did so, I will not comment.

1 part polyurethane's are light duty. They will hold up on rudders but not boards.

2 part polyurethane's, can kill you if sprayed without the proper gear. You can brush or roll in a well ventilated area.

Unless I am trying to match something, I like epoxy for rudders and boards. You have to protect it from the sun (it chalks but you can buff it out) and if you don't like it, it makes a good primer

Interlux VC Performance Epoxy (no biocide) works well on boards and rudders. It is It is a 2 part epoxy made as a racing bottom paint for monohull. You will have to protect it from the sun, it chalks over time. You will need 4 coats. You cannot use this as a primer, it contains Teflon

For cheap, try epoxy with a small amount of high density filler added. This is off-white. If you add a very small amount of carbon, it's black. If you add aluminum powder or a tiny tiny amount of carbon, it's gray. You can use colored pigment, but it will still be yellowish.

DuPont Imron has a water based 2 part polyurethane. It is supposed to be non toxic. It's rated for exposure to salt spray but, it is for industrial use. and it will not be real shiny. I have not tried it. If you try let me know. Most cities have a dealer. They will be listed under auto or industrial painting supplies.