DJ,
Apologies if I'm suddenly stepping in here as if I know what I'm talking about (awkwardly, there is a chance I may be right in this case). To me, the only thing you can do with the part you have, which is beveled on one side and flat on the other, is to use a tool that will replicate the bevel in place of the flat. Maybe this is where you were going and I just didn't catch it from your last post. Granted, to get the seal, you only need a continuous point contact around the circumference of the fitting, but it seems to me that that contact is more likely with mating bevels. Ultimately, I will be surprised if the solution is sand paper.
Richard L.
Sorry, I'm now acting obsessed. Maybe, in lieu of sandpaper, you chuck up the fitting in a drill press and use valve lapping compound to mate the two surfaces.