This guy used blue Dawn platinum, Coke, and lots of scrubbing:
It may have worked as well as the Rustoleum. It would definitely be cheaper, but he sure did a lot of scrubbing, and he used more water than any other method which I have seen, which would not be ideal in a garage.
BuzzFeed cosigned on the Coke, but it worked completely for them:
https://youtu.be/lNFpGClQyAU I am waiting to be impressed:
This guy tried Goof Off, Tide, and Clorox Toilet Bowl Cleaner, and Goof Off worked the best, but I still didn't feel impressed: