With my fenders removed I turned them upside down and first blasted them with a power washer. You might be surprised how much crude will come off. After that I used a putty knife and a 4 inch tape knife to scrape, scrape, scrape. Lastly I used a power wire brush to finish it off. I never used any petroleum solvent although I'm sure it can help. Also, I never completely removed the patina rust, just the flakey stuff, than POR15. Came out great. Mine is a driver, non-show so it's your choice hoe far to go in pre paint prep.
