metallic paint technique ?

You can always get around that by adding 1:1 clearcoat/single stage in some paint systems on the final coat. Pigments are heavier than clear, so the clear settles out on top and you can color sand and buff all the dirt nibs and peal that you want without effecting the metallics. But if you're going to do that you might as well have started out with a BC/CC system. I like to mix the clear on solid single stages though, it adds depth to the finish that you just can't get out of a straight BC/CC.