People keep coming on forums asking how to get rid of tank sealant that has turned to sludge and blocked the whole fuel system. There might be a brand that doesn't do that, but I'd stick with epoxy – it's cheaper and reliable. However, if you're going to line the tank with epoxy you need to take off the tap and all other non-ferrous fittings and slosh the inside with phosphoric acid (dump excess into a plastic basin), wait 15 or more minutes, then rinse the tank very thoroughly with water and blow into it with a vacuum-cleaner hose while heating it gently. When it's thoroughly dry and warm, pour in well-mixed epoxy thinned with a little epoxy solvent (which will evaporate long before the epoxy goes off) and slosh it all round the rusty areas. Pour out the excess through the hole where the tap was. Oh, do wax the threads where the tap goes in so you can re-fit it later. Let the tank sit for several days before putting petrol in it.
There's an amusing video on youtube of somebody trying to weld a tank. It ended up rather more voluminous than he wanted.