[Shop-talk] Source (or alternative) for Phosphoric Acid

[Karl Vacek]

There's another product that's readily available and seems similar to that 
Hirsh product in that it leaves a vinyl-based phosphate coating on the 
steel.  Rust Oleum Rust Reformer.  Just paint it on after removing all loose 
rust, and let it dry.  It's paintable, and in fact should be painted to 
protect it from weathering, but I've left treated things outside unprotected 
for over a year and it's held up pretty well, often perfectly.

I've never used it in a fuel tank, but it's worth testing.

Most any hardware store or home center should have it.

Karl