Theres 2 approaches i tend to do on this
One is deox gel, if any rust pitting left bosh some hydrate 80 on to convert, zinc primer on converted metal, paint of choice and it will slow the corrosion down and get a few years out of it. Its relatively easy and fast to do.
2nd approach is the long winded one
Apply the deox gel until its back to nice clean metal and all rust pitting has gone, a few goes should take it all off which could take several days, cling film it so it does not dry out and ir works epic. Underseal is not the best, i would go for an epoxy primer then top coat, body colour or whatever and look at a dintrinol wax to go over it on the underside bits.
Fuel filler, make sure you get all the rust off the inside as thats where it seems to lurk.
@Ant1 has done a few bits he may be able to advise furthur on the matter ?