I got some aluminum solder sticks and practiced on some scrap alumunium. As mentioned above, very unpredictable results and often the joint was messy despite my best efforts. I burned through thin aluminum about half the time before the stick would flow. I've soldered copper pipe for decades and have silver soldered stainless steel as well as having quite a bit of experience with OA welding but aluminum is beating me so far. I can get a functional joint or fill but the cosmetic results are far from acceptable. Think I'll leave it for the pros.