What can I, in the comfort of my own home, apply to the fronts of the
fenders on this trailer so that the paint doesn't get chipped off by
stones and road debris?

http://www.showroomshineauto.com/images/cub5.jpg

I bought the trailer used (many years ago) and the fenders had been
painted black by the previous owner. More than once I have stripped
them down to bare metal, applied spray primer, spray paint and spray
clear coat, but the paint just doesn't hold up.

Spring is coming and it's time to paint them again. I like the look of
black better than silver, so just stripping them and clear coating
them is not my goal.

Any ideas would be welcome. Thanks!