http://www.solarflux.com/Pages/Productinfo.html
For SS welding I used this, its a powder you mix with methanol and put on the back side of each weld with a little brush. You mix it to a not to think, not to thin paste and after your parts are all mocked up I mark and break it all down to put the flux in.
It leaves a black glassy backing where the weld is, then wash out your pipes when you are done. Looking good, this should move out when your done.