In days past when I did Instant Rust Bluing having a 3 day turn round, on the final clean water boiling I would hang he barrels up to steam dry to keep the heat up in the barrels I used a gas torch but as mentioned a heat gun today would work very well. When the barrels stopped steaming I would put them in a final tank completely submerged in Water displacement oil. This would guarantee as far as possible no water in the rib cavities or anywhere for that matter on a set of barrels. Over time I was never able to find anything better where I could give a guarantee on no water under the ribs.