Sometimes the entire surface is tinned,,sometimes not. No way of really knowing unless you remove a rib and see what their intention was. Then both to see if they achieved it if it was in fact to tin the entire surface.

Some used acid flux which just leaves a mess inside unless thoughly rinsed away.
If used to tin then rinsed thoughly and a non-acid flux for the joining,,you'll not run into the rust problem.

Acid flux gets used for rejoining small sections just 'cause it works so damn good even on a not so clean surface.
A bit of scrubbing with the hot acid flux and the solder will flow.
A polish & reblue leaves the job complete. But after-rust is almost surely to follow and is a nitemare to elliminate down inside.

It's been my observation that most all older bbls regardless of make have at least a pin hole or two in the solder joint somewhere.
You can chase those around trying to solder them up and create new ones I guess. A void or section of the seam that is open is what's needed to be repaired IMHO.

Drying the void with heat after bluing should leave the area safe from rust at that point.
Any red rust that was in there should now be 'rust-blue' after going thru the tank so many time. It just didn't get carded.
Now that you're done,,you have to deside if you feel the need to put oil back into the void or just leave it as-is.
I just leave them alone and solder the muzzle back up.

As far as bluing soln not containing mercury compounds,,that is the reason I went to L/M many years ago,,just to get away from the stuff.
I used to do much more quick rust/hot rust blueing than cold rust.
Nearly all those soln had merc bichloride in them. Way too many years of handling that stuff for me.
Even the 'new' Herters Belgian Blue from Brownells has mercury in it from what I can see stiking it onto warm brass or copper. That suprised me in this PC environment.

For quick rust I use Mark Lee rust blue solution.
Laurel Mtn for cold rust. I think L/M is mainly ferric chloride.

They both work well for me and I'm passed the point where I want to do alot of experimenting anymore.