Place to put finishing, soldering, etc.
Soldering to stainless steel
Stainless steel is actually a piece o’ cake to solder, provided you use acid flux in liberal quantities. Look on McMaster-Carr for it. The recipe is also online in places. It contains concentrated HCl, some zinc chloride, and some ammonium chloride. These components create a highly oxidizing / dissolving environment that strips the protective oxides off the metal. Use sandpaper to pre-roughen the joint areas. Heat the parts. Spray on flux, trying to control the extent it covers (or the solder will follow it all over your parts). Use glass wool or a stainless needle as an applicator-nothing organic or you’ll find it makes a tarry mess. Add solder to the joint. Usually it helps to re-flux the joint a few times after the solder is molten, just to help homogenize and clean the joint. As soon as the solder sets, immediately wash the parts in hot water to get rid of the corrosive flux residues. Further cleaning in CLR (phosphoric / oxalic acids) will remove residual oxidation from the surface of the steel and return it to its usual stainless color. Do not use acid flux indoors or near metallic objects you care about.
https://fusor.net/board/viewtopic.php?t=3667
See also video by rotoforge
Type 3 anodization - boiling water treatment seals pores