My method/process
1) Clean oily parts with degreaser.
2) Bathe in warm caustic soda solution to remove all traces of paint.
3) Overnight electrolysis to take care of any rust.
4) High pressure hosing or a little elbow-grease with a stiff brush to scrub of oxides.
4) Gentle heating with a blow-torch prior to spraying.
5) Use good paint. I just use spray cans, but have had excellent results with Wattyl Killrust.
Craig