Citric Acid, seems to be working fine. It's a bag of the stuff I've had since I did a coat of the garage floor. It was to clean all the grease and oil off the concrete before application, since it was new I never used it. Thought I'd give this a try...
Never heard of using that before. If you have some stubborn spots, wash thoroughlywith soap and hot water, rinse, rinse, then put in a trash bag and spray liberally with easy-off oven cleaner. Tie up the bag and let it sit in the sun for a couple days. It will eat all the crap off of there. Then wash, rinse, rinse, soak in vinegar and water if rusty, scrub, wash, rinse, rinse, rinse, season.
Oh yeah, add some pics.