If you are asking me: I have no f*cking idea.
Haha, okay.
I was looking around online at what will passivate (get it into your bed and accept your solder) stainless steel.
I found that really lots of acids do the trick. I had heard of lemon juice being used as an emergency flux, and sure enough, it can be. However I would think that rubbing lemons on ss wouldn't work because it is so finicky. Fortunately I have some citric acid hanging around. Ill try cleaning up the SS with the citric acid as its less hazardous than the muriatic acid (being hydrochloric acid.) The muriatic acid also likes to keep eating away at SS so you need to really wash it well after you use it. Most fluxes for SS are phosphoric acid, but the only supply of phosphoric acid I currently have is naval jelly, which is on hand.
I don't want to spend 10 dollars on what will be a single use item, so if I can just get the washers, copper, and ball valve and get that soldered together I will be golden! I will test it out tomorrow and report back with results!