A couple things come to mind. Never add water to acid to dilute it. You can add the acid to the water carefully, but the other way can lead to uncontrolled reaction and injury. A phosphoric product I use is called OSPHO runs around $20/gallon. Another product is EXTEND made by same mfg..
Another way to remove rust is electrolytic removal covered in threads here and in tech tips. After using that, you could then etch parts with acid, wash and wipe dry or use scotch pad and then paint.
