I usually don't bother with rust converter but figured I'd try it out. It's actually interesting to see it at work, it's turning it black. Might have to go back and put more as some rust spots did not quite convert but maybe it will take a while longer. With all the salt they put on our roads it's inevitable that it will rust again, this is mostly just to prolong it so it does not rust right out. If I had a proper garage/shop area I would do under the car too. My garage is way too small to work on a car though. I don't even use it for parking in.