MY 2006 WRX aluminum control arms were covered with both a black paint, and a oily like substance making it difficult to soda blast.
My salutation was to spray "aircraft paint remover", in the spray can. and than Brillo pad the entire unit.
Tip for using the paint remover, the directions said to spray and let stand for 15 to 30 minuets. I sprayed it on and left it for about 10 to 15 minuets and
removed the paint residue with paper towels and than repeated the process. After witch I rinsed them off in the sink, and used a Brillo pad to further clean the services.This process cleaned about 95% of the problem areas. A WORD OF ADVICE - WEAR RUBBER GLOVES. grayghost