Tractor Supply CO. has a product, (Sterosol Milkstone Remover and Acid
Rinse,SKU # ST11), that has the same active ingredient, (Phosphoric Acid), as all the other rust removers and is about $12.00 a gallon. One gallon allows you to make 10 gallons of cleaner. Only stores that have dairy farms in their area carry it, but I would imagine any store could order it for you.