hey there. My bike wasn't powder coated; it was painted by the good folks at M Steel cycles up in Newcastle upon Tyne. They did the whole job - particle blasted off the old paint then acid etch primer, 2 coats of colour, lasercut decals and a coat of clear laquer to finish. It looks like they must have simply pushed something (cotton wool? masking tape?) into the holes to stop paint getting in the threads. The washer edges are painted but it still rotates and operates as designed.
What colour are you having it done? Always nice to hear of a full on resto with a new paintjob!