With a natural finish about your only option is to media blast it clean. If you're not in a big rush for it and it can sit at the shop until they're doing another job like it you'll probably pay $20. If ya gotta have it now expect closer to $50.
If it has to be done on the car, well good luck. I don't know of any chemical that with safely or effectively get it clean. If anybody sells something to do the job it will be http://www.eastwoodco.com/. Lucky for you they're also in PA. Good luck.
Thanks for the pic, jedimario.
"Everybody believes in something and everybody, by virtue of the fact that they believe in something, use that something to support their own existence."
Frank Vincent Zappa, 1940-1993