I don't think muriatic acid is going to give you the results you want but knock yourself out. A 4:1 mix will take forever to eat any significant weight away. In order for muriatic to even begin to work the parts have to be bare metal - free of everything: paint, oil, etc.. The acid will leave a rough, porous surface so you're going to end up putting an equal amount of weight in bondo back on to get a decent paintable surface! Personally I wouldn't want to be anywhere near that vat - the fumes from just a quart bottle will damn near kill you!! Just out of curiosity, what's your disposal plan - being from the land of tree-hungers and all ?!?!?