I think they get it from Tractor Supply or Rural King. It is a hardener for one of the farm implement paints and is on the shelves with the Rustoleum. Sorry, but I don't know the ratio.
I think it's mentioned in here though.
This I searched that thread for "Hardener" and got nothing. Nice pics though. I Googled "rustoleum hardener" and there is quite allot out there. I suggest the threads on the HAMB.