Powdercoating is a heat-curing (or rather, heat-flowing) coating. When applied, the powdered part is put in an oven and baked at around 400 degrees, where the powder literally melts and flows out over the part.
As you might imagine, this is not a suitable application for an exhaust pipe, which should- not just "can", but should - see in excess of 1100 degrees in service. The powder will burn off in the first flight.
The cheap-and-simple fix is of course some flavor or another of "hi heat" spray paint meant for barbecues. It has to be periodically reapplied, but it'll work.
A more expensive solution would be to send it off to a company that does exhaust plating, like the so-called "Jet Hot" coatings, which are a proprietary metallized ceramic applied with a flame gun. (Think "spray MIG".) They're typically not black, but the ceramic aspect acts as a heat reflector/insulator anyway.
Something in between, you might try one of those bake-on firearm coatings, one of the ones with a heavy moly-disulfide content. They're sprayed on and cure at about the same temps as powdercoating, but the cured coating generally (depending on brand) has a fairly high heat tolerance.
