Did you ever wax after you finished? These paint jobs really need a good coat of wax to protect them, unlike factory paint jobs where the clearcoat provides most of the protection.

Unwaxed paint with no clearcoat acts almost like a sponge for dirt and dust and road grit and bird poop and everything else a car's paint job is exposed to.

I'm finding that myself, since I park under a tree where birds roost at night. When they crap on my white paint, the stuff can in some cases become almost etched into the paint. I have yet to find a way to remove the really stubborn stuff. About all I can think of is to put another coat of wax on and hope that helps.