Removing Rust in a 6 gal fuel tank

[bksah2]

I picked up a older metal 6gal tank at a yard sale ,,tank has a little rust inside ,,should i trash it or is there a cheap way to clean it and coat it so the rust does not return ,,, when i would clean a aluminum tank i would just throw in some nuts and bolts and acetione shake and clean never had to coat the alum. ,,,but if i do that to the metal tank without some type of coating the rust will return,,, remember CHEAP is the key