I do quite a bit of rust bluing and would not consider rust bluing an action and barrel together, for the reasons previously stated.. Weeping of oils/lubes.
Getting rusting solution and water between the receiver threads and barrel threads is a great recipe for rusting, as it would be very difficult to get all the water out. After a few months I suspect that the barrel and receiver may become rust welded together.
I highly recommend a lube on the threads when installing the barrel; to help with installation and to keep rust from forming. Since I work primarily on antiques, in lieu of anti-seize, I use a more traditional mixture of 50/50 cosmoline and yellow axle grease.
I always test fit/test fire complete assemblies prior to final finishing.
