First of all, the initial gelcoat from the mold is generally pretty tough. But if you need/want to paint
for some reason: For a job I would be happy with, I'd want to use something like a sprayed-on (or rolled and tipped) two-part linear polyurethane. They're very hard and durable and shiny paints. You do have to wear a supplied-air respirator when spraying them, but for roll and tip you can use a good quality cartridge respirator (they are quite useful for many projects around home and trailer anyway).
There are one part paints (which may be polyurethane but are not the same as a two-part LPU). These are a bit less expensive and less challenging to put on, but on the negative side, they are not as durable and the shine doesn't last nearly as long.
Prep, however, is the same amount of work for each one...
And the prep is the key to a good finish.
For those units, I might want to remove them and take them out into the shop to paint
(then you can re-seal your rivets, or perhaps eliminate some), especially if you are spraying.
Really, I always tell myself that painting
is days and days of prep, and side jobs that the prep suddenly alllows (i.e. rivets, painting
behind flanges, running wires while they're out...), followed by days more preparation and a hundred trips to the catalog or Internet or store, followed by a bit of last-minute prep, followed by -- incidentally -- a bit of painting