Last year I had to do some soldering on a rear pan of a maple syrup evaporator, IIRC it was 304 SS. A large cast bronze flange had separated from the pan at the solder joint. I was concerned with using the correct food grade solders, as this joint is in contact with the partially boiled sap, even though it was a tiny area of solder only the thickness of the solder in the joint. I contacted the Leader Evaporator Co. of Rutland, VT. They were extremely helpful, even telling me the best methods of heating the joint and prep. The solder I was told to use was, IIRC, the Harris Stay-Brite solder. I don't remember for sure the flux I used, but I think it may have been Stay-Clean, a Harris product, as per the recommendation of Leader and a local plumbing/HVAC supply house. As per Leader's instruction, I used a propane torch on the casting for general heat, and a well tinned soldering copper for the tinning and on the SS side of the joint for the actual soldering. Leader still uses soldering coppers in their factory, from what I am told, however some of the new pans are TIG welded. I applied a relatively thick layer of solder to both surface for tinning, and then heated the surface from the back, and then wiped the excess solder off with a clean, wet rag, then immediately applied more flux to the surfaces, then fixtured them together, and heat, while applying solder around the joint. When cool, I neutralized and CLEANED EVERYTHING first with a wet rag a baking soda, followed by a thorough polishing of the joint and adjoining areas steel wool and those "S.O.S." pads. Worked great. I am not positive about the exact name of the solder I used, other than it was a lead-free, cadmium free, potable water, silver-bearing, also recommended by a local plumbing/HVAC supply house and Leader. The biggest problem was another nearby seam, so I had to clamp some large bars, "heat sinks", on either side of the SS pan side, and use wome wet rags to keep heat at that joint to a minimum. Hope this helps, sorry for the lengthy post.