muratic acid works on rust, not so much the sealant. also make sure to dilute the acid. pool supply stores carry it at about 19 percent mix. use 75 percent pool mix / 25 percent water. and yes, if it touches the paint, its gone. you have to keep an eye on it though, mine was stripped of rust down to bare metal within 10 minutes. it will eventually eat the whole tank if left by itself. also, I did mine while standing in my pool with the tank submerged. if any acid escaped the filler lid, the water ensured it was diluted to save the paint, it worked like a champ.
make sure to rinse it with water to stop the acid when done. and dry it with a blow dryer in the filler neck IMMEDIATELY because it will flash rust within minutes.