For cleaning out rusted gas tanks, we like to use ordinary white vinegar with some loose screws or some chain to loosen up the rust. Fill it up all the way to the very top (important). Shake it around for 5 min or so every few hours over 24hr. Do leave it longer than that, otherwise you risk eating all the way though.
If its clean it needs to be rinsed immediately with WD-40 or Marvel mystery oil to prevent flash rust and then a second rinse with clean gas.
If its still dirty try the vinegar again for another 24 hours and check.
We don't generally recommend coating or sealing the tank if it can be avoided, but if you're going to anyway we recommend Caswell tank sealer.