Fellstone

FS 36375 is Light Compass Ghost Grey. Hu127 is described as US Ghost Grey. I'm not able to find any reference to FS 36275, so I'd be inclined to say typo from the original source.

I've had good luck with Mr. Surfacer 1500 thinned in the 50% range with Mr. Color Leveling Thinner using a 0.35 needle at 20 psi or so. When I thinned too far (say 2/3 thinner), I had some issues with the primer peeling after masking, but I can't definitively put it down to the thinning ratio -- given the alchemy that is airbrushing, it could have been the phase of the moon. Can't speak to the retarder, though I believe that Mr. Color Leveling Thinner is pre-mixed with a retarding agent.