Not really sure how else to word the title.
Called a local gunsmith about getting some barrels blued. The barrels were previously factory blued (black oxide), but where then bead blasted and then hydro dipped. Not every area was bead blasted, just the outside area. The sights, the barrel retaining lug, and breech were all taped off before blasting (as well as the bore/chamber) and those areas still have the factory black oxide bluing.  (found this after removing the hydro dip with acetone)
The gunsmith told me that there are many different formulas for hot bluing, and unless you know which one was originally used, it all had to be removed before being dipped in the new hot bluing.
He said if all of the the original bluing was not removed, that where the original bluing was, would come out as red rust.

I know nothing about hot bluing and I do not disbelieve him, I just want to hear from some other experts. Is what he said true?
If so it is not really that big of a deal. I experimented with some small random spare parts from my junk box that were blued and soaked them in vinegar and it dissolved it no problem. So taking it off should not be a big issue.

Tired searching a few different terms online and found a thread on a S&W forum where it appears, based on the replies, that removing the old bluing is not required.

http://smith-wessonforum.com/s-w-smithing/558650-bluing-without-blue-remove.html