I have had two doubles, an L.C. Smith Ideal grade and a Lefever F grade that had been hot blued long before I acquired them. My experience is the same as tanky had with his 20 ga. Sterlingworth. The ribs remained tight and solid. The solder joints never showed any sign of deterioration.
In spite of that, I would never recommend hot bluing a soft soldered vintage double. We know for certain that hot bluing salts are corrosive to tin/lead solder. We also know that many doubles have imperfect rib solder joints that can allow these salts to become trapped in the void between the barrels, and the salts willl eat away at the solder and the barrels.
But solder is not porous. In fact, it is a very good sealer, and that is why it has been used for decades to perfectly seal plumbing joints which continuously hold liquid under pressure. So if a corrosive agent such as bluing salts is completely rinsed away and neutralized, any potential damage is stopped then and there. On the other hand, if any bluing salts at all is trapped between the ribs, or in a small void in the solder joints, it will slowly attack and destroy the joint.
Think about this... there are a lot of old shotguns that have sections of rib pop loose. The vast majority have never came within a mile of a hot blue tank. The joints fail because of physical damage, or more likely, because they were poorly soldered in the first place. The greatest cause of solder joint failure is contamination and poor preparation and cleaning of the joint surfaces. Other factors are use of the wrong flux, or not using the correct alloy of solder.
