Well, if you've made sure to prime everything, I've found that I like this product called Rub'n'Buff.
You can find it at Michael's for usually around 3 dollars a tube. BUT THAT STUFF LASTS FOREVER. THERE'S NO END TO IT.
What one usually does is put the rub'n'buff on top of a painted surface. Here
is a sample on what someone's done with metallic paint and some rub'n'buff.