Removing Lacquer
use an aircraft grade metal stripper to take the lacquer off. If you're lucky the coloring is in the lacquer and will come off with it. However, if it is a separate coating, you may have more work to do.
On some older horns a very hot bath may remove some of the lacquer There are warnings about hot water because it may destroy the Lacquer.
The consequence of too hot of the water was discussed.
I would think twice about boiling an instrument. There's a difference between a very hot bath, and 212 deg F. Especially on an older horn, you're most probably going to pop a solder joint or two (or worse) if it's at that temp for even a few minutes.
I don't know about what other negative effects boiling may have, but I think that solder melts at +350 degrees. And while you can melt solder by buffing too hard, I would be surprised to learn that you can get it hot enough with water to melt.
I think the problem with boiling isn't a risk of solder melting, but a risk that the different coefficients of expansion might lead the solder to crack as the brass expands due to the heat.
The expansion of the brass is what you're counting on for removal of lacquer, for example, with a hot bath. The CTE (co-efficient of thermal expansion) is exactly the problem. The difference between the brass, regardless of plating, compared to the solder is miles apart. Add in any stresses imparted on the horn during assembly and it's just aching for a reason to separate.
Current lead-free solder (95/5 silver/tin) melts at about 360 and takes just a little more than that to flow well. Older leaded solders (50/50, 60/40) melt at 320-340. Still not enough to melt in boiling water, but that much less protection against popping braces, ferrules etc.