Re: spray cans
Yep, spray cans are best for painting small surfaces. Large areas need a spray gun. If you don't have compressed air, an airless sprayer will work fine. It will allow you to thin the paint enough that it will stay wet until you are done with a large are so there willl not be stripes from edges drying. Keeping a wet edge gives it the overall smooth look.
If you are working for a auto quality finish, get some good spray equipment.
If you are looking for something that will look fresh, hold up for years, then thinned Rustolium in an airless sprayer (about $45 to $75 ) will do the job. There are also urathane paints you can buy by the quart that you might want to look into. Stop at a paint store (not the Depot or Lowes, etc) and ask what they have that will hold up above the water line.
PS, I haven't tried roll and tip, but lots of folks on this forum have posted photos of amazing results using that method.