Yeah, Tim has it right Carl. Lacquer thinner will work too. The scuffing is really important for mechanical adhesion. Use the scuff pad so as not to introduce scratches.
I think why they do that is alum after welding can have discolorations and splotchy areas and they want to put their best foot forward and make it look uniform. I wonder if it would also be good to use a pressure washer (take it to the carwash) and really get the fins and hard to get areas with soap and water. Rinse clean, dry WELL, then Naptha based (PPG DX-330) or a good metal prep cleaner. What a pain! ANY small place you dont get scuffed and thoroughly cleaned will not hold paint for very long. Good luck!
