No brake clean, unless you're the type to play with loaded guns too.

Acetone or a dedicated aluminum cleaner from the weld supply shop. A clean, stainless brush. Aluminum, especially cast, will absorb whatever fluid it's in contact with to some degree. You may find that you have to get it wet with the torch and clean off the poop and crap that comes up out of the metal multiple times before you can get a bead to run. Using the propane torch also helps.

If it's cracked through a bead or a thick piece, V'ing the crack will help.