Removing rust from inside a motorcycle petrol tank

Vinegar is good for light surface rust. Brick acid better for heavier rust but can eat though the good metal is left in the tank too long. Best to try and loosen any rust first. People often advocate a handfull of nust and bolts shaken vigorously but I find broken windscreen glass better even if more messy. Red Hermatite thinned with cellulose thinners make a good, cheap sloshing sealer. Just ensure you leave it long enough to fully harden before adding fuel.

Paul H

Chemodex rust and scale remover is the business for this sort of thing. Por15 Metal Ready would probably do it too.

Citric acid. About £5-6 for a kilo delivered, makes loads of liquid. Works faster if made with hot water, did some bits the other day, all rust gone in around an hour with a hot solution. I chucked some bits in an old saucepan and a bit of old ply on top which kept the heat in well. Otherwise I usually leave stuff overnight in a cold solution.
Very effective, cheap, easy and safe.
According to someone on another forum it's the main ingredient in Deox-C, but I've no idea about that.
Also makes millscale into a slime that wipes off black mild steel.

It'll probably be rusty up top as well where you can't see, condensation sitting above the fuel. If it's rusted now it'll rust again, unless resin'd with something like the POR resin.