clean with acetone and use a blacking / rust solution then make a mist cabinet etc.....after 45 mins remove and dry completely and then rub with fine dry steel wool ( immerse wool in acetone then squeeze and let dry to remove oil ) then put back in mist cabinet and repeat etc
edit.... you can even dangle in boiling water for 20 mins then dry etc then do as above https://www.youtube.com/watch?v=O4QOcRTZv2Q