Hello Dorie and welcome to the forum!
My first suggestion is to use the search function here and look for past discussions. Full answers to your questions would be beyond my abilities, but the collective wisdom on the forum will soon kick in. Gelcoat is the standard finish on these trailers. Some have mentioned that it is actually just more of a release agent in the molding process and therefore not a paint
About the tar and grey spots: I'd suggest finding an inconspicuous area and experiment. Tar and spots may come out with alcohol, but more agressive chemicals may be necessary such as acetone.
More agressive still may be abrasives, such as automotive polishes, then rubbing compounds. Beyond that perhaps wet-sanding with 600 sandpaper (then maybe 1200 grit then rubbing it out with rubbing compounds and polishes). If you follow automotive rub-out practices and are careful not to sand away too much of the gelcoat you may yet come up with a white and shining surface. Finish with a good wax.
That said, gelcoat is softer and more porous than automotive paint
. It seems to stain more easily and hold dirt. The gelcoat on mine needs some repair, and I am considering painting
it eventually. I have made some tests with Interlux Brightside enamel (from marine suppliers) and it shows great promise aside from being capable of being applied with a brush or roller and still come out shiny and smooth. Great stuff.
Cracks: it all depends on how deep they are and that may be hard to determine. I have some hairline cracks in mine and I intend to use a burr on my Dremel rotary tool to gouge them out and then fill them with epoxy, followed by the usual sanding and rubbing out. Another good reason to paint
instead of trying to match and apply the gelcoat.
Congratulations on finding a Burro! If you are like most members on this forum you will soon come to appreciate what you have and what you can do to customize and improve how it functions for you.