I had one years ago that was greasy as hell everywhere...it didn't rust at all but I never thought I'd get the oil off of it enough to weld it up and paint it.
Washed everything off as well as I could at the car wash and then ended up using a rosebud tip on a torch up through the middle and baking all the crap out of it...cleaned up a lot easier...
It still only has primer on it...I'm not painting it until everything is ready to be assembled...