Muriatic acid is one of my favorite chemicals.I deal with a lot of rusted parts,and muriatic acid is the best,and cheapest,rust remover I know about.I probably use 20-30 gallons of it a year.As long as you know how to use it safely,you are fine.It will not harm the tank in any way.In the few cases where I've had pinhole leaks show up in a tank that I cleaned with muriatic acid,the tank was so far gone that it was junk anyway.If it's a super rare or otherwise irreplaceable tank,the holes can be repaired by welding.I repaired an early '60s Honda tank that had several small leaks along the bottom seam by welding a bead all the way along the inside edge of the seam.If it is a larger hole,I've seen sheet metal plates welded over the hole,and then feathered in with bondo.Just remember,you have to get rid of ALL the gas and gas fumes before welding on a tank.Otherwise,you might as well be welding on a bomb.Anyway,after using muriatic acid to clean the tank,I recommend a thorough rinse with water,followed by a thorough soaking in WD-40,immediately,before the tank dries and surface rust forms.WD-40 displaces water.Seal the filler and petcock openings to keep dirt out until the tank is ready to use.Pour out any excess WD-40,and pour in premix.What little WD-40 is still in the tank won't hurt a thing.Peace.Jerry.