I refinished a glass Pfeiffer trailer for a cirrus about 25 years ago
with Imron. The trailer is still around and sits out in the sun at
Ephrata every summer. The paint has done very well and still looks
great. The Pfeiffer trailers did not have a gel coat finish so cracks
from underlying gel coat were not a problem.

Most polyurethanes use very nasty isocyantes for curing so you should
have professional spray equipment with outside air supply etc. Best
way around this is to do all the prep and then arrange with a local
autobody shop to do the actual spray work. They have all the
facilities and do it on a daily basis.

Prep the gelcoat with 220 grit wet to give the polyurethane some tooth
to grab onto. After sanding wash down with clean water (no soap or
detergents) and completely dry everything taking care to blow water out
of any crevices.

If there are existing cracks or crazing in the gel coat they will
eventually work their way up through the polyurethane. Only way I know
around this is to completely strip the gelcoat which seems like way
more trouble than it is worth for a trailer.

Refinishing with gelcoat in my opinion would just compound the existing
problems. Polyurethanes have much better UV resistance and are not as
brittle. It is also easier to find a body shop to apply them. Done
correctly the polyurethane won't need any post spray work (sanding,
buffing etc.) at all and you'll be set for years to come.

Best of luck

Craig Funston