Sanding takes way too much time, in my opinion. Powders are good at hiding small imperfections because they are typically applied thicker, where as liquid coatings are applied thinner. Also when curing, the powder flows out and self levels, which helps to fill in moderate scratches. If you are working with cast aluminum, say a intake manifold, i would pick up some 80 grit greaseless compound and spiral sewn wheels. They cut much faster than paper and can be used in a drill with a mandrel....thus saving your arm from falling off
. You might also want to pick up some felt bobs to use with the greaseless, and some sanding rolls/tapers. With these products, you should be able to knock down casting quickly. After you've gone through this, give it a quick thorough blasting with with a bead or oxide, or whatever you happen to use. This will remove residual greasless compound and other surface contaminents from handling it and what have you. From there continue with the rest of your powder coating process and you will get a nice, seemless, slick look. Good luck.