I had a similar experience when I repainted my old TR250. The chrome badges had some kind of "paint" on them that nothing seemed to be able to remove. I'm sure there must be some sort of color "paint" that hardens like porcelain and that might be what you have.
Try an experiment. Paint two test pieces with enamel. Cure one in an oven at around 400º and let the other air dry and see if there is a difference. I know it's not the same paint as what you have on your old truck but I think you may just be surprised at the results.
