That's one gorgeous holster. I don't know how you got that finish either, but keep doing it because it looks great.
I know what you mean about the foamy resolene. I have to wipe mine down the same way, and it's easy to get a blotchy or uneven finish. An airbrush is one solution to foamy brushed on resolene. I really like the airbrushed resolene/water finish, when applied evenly. Just be sure to wear your respirator, as with any sprayed product. I really can't see the glossy/satin spots in your pictures... Besides, after being worn in the pants a while I don't think you'll notice it anyways.
I can't really recommend anything on the exotic gluing... I've never used Barge or exotics. I have some Weldwood contact cement, and Tandy 'tanners bond craftsman contact cement'. The Weldwood goes on much thinner and evenly, IMO; But I think the tanners bond stuff holds better (it's just more bulky and leaves a more visible line on edges, in my experience anyways). I'm not sure which one is closer, if either, to the Barge formula.
I think you'll be your biggest critic in regards to this piece. This holster looks great! I've got a request for a left handed IWB coming up. I'd be very happy to have mine turn out nearly as nice looking as yours here.
Nooj