I've never tried to solder aluminum, I've just mig, tig, stick or welded with an oxygen acetylene torch and for those processes the operative word is CLEAN ,clean and clean some more.For mig and tig the use if a pure inert gas such as Argon of Helium is required for stick the electrodes are flux coated. after cleaning of the weld can be a bother I normally use a rotary SS wire wheel. For torch welding I use the stick electrodes and a low neutral flame slightly on the yellow side towards carborizing.
The HVAC guys routinely solder Aluminum to copper but I've never tried it so can not explain their process.
When I was a kid an Uncle of mine explained how he as he called it welded pewter buy making welding sticks out of doner pewter buy melting it down then removing one side of a piece of corrugated cardboard to make sticks somewhere along the way he used a judicial amount of a paste he made out of oxalic acid and wax he made form honey comp other than that I do not remember much. I watched him do it a few times but was too young to try it for myself I do know that he could mend a broken pewter heirloom so it looked like it had never been repaired