Sorry for the slow reply. I ended up trying the 85420 stuff. At first it seemed to work. No leaks after a few flights. Then, after sitting for 2 weeks I went back and found it was leaking again. Both at the bottom fitting and the sensor fitting. It appeared to have eaten away at the blue sealant as what was left was white-ish. I removed it again and have re-cleaned and re-sealed it again using the same stuff (i don't have anything else.). So far on a pressurized bench test it's holding after 24hr. I'm going to leave it for the week. However, I'm a bit concerned the threads are damaged in the aluminum. This time I went for broke with tightening the fittings and they seemed to get a full extra rotation. So either I was too safe previously or I just screwed them up for good. None of the fittings in my header tank have suffered the same and they are using the original other permatex sealant which hardens. If it leaks this time I'm removing it and just going right to header tank.