I've used Norrel's Molycoat extensively. Most of the surfaces have been parked. I did however do a Mossberg 500 shotgun that was blued. I didn't have any issues at all with adherence. Make sure you degrease and preheat.
YMMV
Bomber
ETA: From their website molyresin.com
Moly Resin™ will adhere to all metals including the following: aluminum, carbon steel, stainless steel, sandblasted and glass beaded metal, cast iron and aluminum, titanium, copper, silver solder, blued or parkerized finish, anodized, nickel and chrome plate, and many plastics, etc. Note: nickel and chrome plated surfaces should be abrasive blasted to allow the best adhesion of the Resin. For all metals, best overall results are achieved on freshly sandblasted surfaces.
So it does to stick to blued surfaces which is my experience. They do however say sandblaster surfaces give best overall results.