Stat clean is what I used, with silver solder. Use a soldering iron, not a torch. A torch can work but an iron is far better. In fact 3 irons. Have an oven heated by the flames of a turkey frier burner and get 1 red hot, use it, then put back in the oven and use the next. Keep rotating irons. To clean an iron, file to shinny, then heat it red hot and rub it on some "sal ammoniac", then the iron will hold the solder well, and deposit it on the joint being soldered. If you must use a torch, don't hold the flame in 1 spot, keep it moving. If you can apply the flame from behind, it works best. Get it just hot enough to melt the solder, don't overheat. I have successfully done it both ways, the iron method is far better. 1 iron will work if you can't come up with 2 or 3. (I have over a dozen, different sizes and shapes, sorry, not for sale.)
Dave Klish, I recently ordered a 2x6 wood fired evaporator from A&A Sheet Metal which I will be converting to oil fired
Now have solar, 2x6 finish pan, 5 bank 7x7 filter press, large water jacketed bottler, and tankless water heater.
Recently bought another Gingerich RO, this one was a 125, but a second membrane was added thus is a 250, like I had.
After running a 2x3, a 2x6, 3x8 tapping from 79 taps up to 1320 all woodfired, now I'm going to a 2x6 oil fired and a 200-425 taps.