The trouble with solder is that the metal has to be CLEAN for it to stick. Brazing alloys are a little more forgiving, as they usually require the base metal to be dull red, which usually means the rust is burnt off.
Yes of course that's true, but welding any dirty metal is not advisable, either. For what it may or may not be worth, I worked as a commercial welder...but that was over 50 years ago when (thankfully) welding was taught more bottom-up than it is today. I also at that time had certifications including underwater and 6 hours of advanced university classes in the subject. I've mentioned silver bearing solder and a flange edge vise grip here many times. It is the fastest and easiest way I've ever found to install straight seam patch panels with minimal heat, no warping, no burn-through and the ability to re-position almost infinitely if needed. If I had any of that work needing to be done and a video camera I'd show the forum what I mean. Stay-Brite works at 430 F and has a shear strength of something like 10,000 psi. You can install patch panels using a MAPP torch if you want, but I enjoy the speed and more localized heat of an O-A rig. Plus if any solder gets outside of the seam you can dress it down easily. HVAC boys and commercial plumbers know all about the product and use it all the time, but for some reason I think most people have never heard of it.
