The flutes should be coated, then the surface around the flutes should be polished IMO. Over time, the outer surface will have wear marks from working the bolt so I don't coat the outer bolt body. It's real easy to wipe down the polished area with some Corrosion X, no rust issues.
This bolt was coated with KG guncoat and then polished giving the flutes a nice contrast. Be careful when blasting, although the bolt lugs were coated in this pic, I was careful not to blast the bearing surfaces (the coating was very thin on the bearing surface of the lugs - not enough to cause any issues)
MtnHtr