[ales]

It's rather simple - if you sand/polish metallics that aren't clearcoated, you're disturbing the metal or pearl flakes. A clearcoat protects the particles and accentuates the metallic effect of the base coat and makes helps achieve the deep shine, but you don't sand the metallic, you sand and polish the clear. This is just the procedure for these paints, there isn't a metallic or pearl car on the road (or rally stage) that isn't clearcoated. Of course if your way works for you, go with that, but the general consensus is that metallics and pearls need clear.