Nothing that comes in a spray can (except maybe cleaner) should ever be used on the outside of your trailer. Any spray can
paint will fail within a few years and you will have a mess.
BUT, I believe that you can get custom tints both in Briteside as well as Rustoleum TopSide Marine
paint. Regular rustoleum should be left for lawn furniture, fences. etc. If you have used Rustoleum spray can primer already, at the very least, I suggest recoating with their Marine primer, if not removing it entirely. Many paints and primers just don't bond to
fiberglass and/or gelcoat very well
And yes, it will take at least two coats of Briteside for a good white finish. Had the same issue when I painted my Hunter Compact-II. BTW: I hope you used Interlux "Pre-Kote", it forms a polymer bonding link of some sort with the color coat.