Abe wrote:
....
Bob's correct. A dielectric union is the only legit way to mate copper
and galvanized.

Actually, I saw a new one (to me, IANAP) relatively recently -- similar
problem; old galvanized in very difficult to access location in service
tunnel in old church building. Local plumber had a compression-type
fitting designed for the purpose of threadless connection to galvanized.

I don't recall the specific brand name but it works and is designed
specifically for the purpose. It is comparatively pricey relative to an
ordinary dielectric union, however, but in a situation as we had the
labor to manage to thread the other or remove even more back to a next
joint would far outweigh the cost of the part. (Of course, one of these
days, the whole thing will have to come out, but that's a story for
another day; hopefully after I'm no longer on Trustees ).

So, I think the answer to the original question is "it depends, either
technique can be ok if it's the right product".

--