I am supposed to specify a powder for coating alloy wheels. Will an epoxy-polyester film give me an orange-peel-free surface at 150-200 microns? S.W.
It depends upon the particular formulated properties of the particular powder coating you are evaluating. Not all epoxy/polyester powders have the same smoothness at a given film thickness. The formulator can increase the flow characteristics to allow the coating to flow more smoothly or go the other way to have it gel faster leaving more orange peel.
However, your coating thickness of 6-8 mils (150-200 microns) may be a problem. At that film thickness on a cold part KV rejection takes place, causing a rough surface like orange peel. Preheating the part and using special application gun features may help to overcome this problem somewhat. However, you should test apply the coating just to be sure.
Beyond the smoothness part of your question, I have serious concerns for using epoxy-polyester hybrids on wheels. This type of coating has poor UV resistance and will chalk fairly quick. Unless you are using this material as a primer surfacer, I would recommend that you use a TGIC polyester, pure polyester or acrylic, all of which are UV stable materials and will not chalk in sunlight.