An inexpensive phosphoric acid is Klean Strip® Phosphoric Prep & Etch. About $16.00 a gallon at Menards. I also use feed grade molasses from Tractor Supply where 1 gallon costs $10.00. One part molasses and eight to nine parts water will give you a 9 to 10 gallon rust removing solution. If you build a vat with lumber and a plastic liner (or buy a kiddie pool) you can have a 50 gallon solution for $50.00. How large are the parts?