can't say I've noticed any tanks that had what appeared to be sealer, most I've found had cork gaskets that came off easily .... I wouldn't worry much about leaks from the sender and haven't used any sealer there, the only time liquid will actually push against the top will be when you fill the tank til it's up the filler neck, bad practice as temperature expansion will run it out the filler and ruin your paint .... fumes are the problem and the fumes some complain about in the cab are most always from the hose connections on the fill and vent tubes, make sure those fit properly and are tightened well - as for "the pressure getting really high really fast", how could that happen? the tank isn't pressurized, when the fuel gets sucked to the engine, it needs air in from the vented cap, a plugged cap would make for a negative pressure that'd pull air in any leaks and/or cause fuel starvation
Bill
