I dont see any reason you cant use muriatic acid on the inside of your tank. Just dont leave it very long it dosnt take much time. Rinse it out the use a baking soda and water solution to neutralize the acid. then rinse it again. Of course it will "flash" rust as fast as it dries. I uses it all day yesterday on my inner fenders and fenders with no problem. First time I did my fenders The acid seeped out of the seams no matter how much I rised. So I use the baking soda and water and wiped it down well and sponged it into the seems and cured that problem. Now with all that said Eastwoods sells a kit just for cleaning and sealing the inside of gas tanks. may want to look into that.
Now if its alunimum dont even get the muraitic acid close to it!
Good Luck!
