Ospho lists the active ingredient as Orthophosphoric Acid, which google suggests is just plain old phosphoric acid. I use Kleenstrip Etch and Prep, which is basically the same. Less than $20/gallon from Home Depot, the last time I checked. It works great to derust. You can cut it to dilute a fair bit too, if you want more volume. Evaporust is more mild; if you are derusting a painted part, and want to save the paint to the extent possible, I'd use Evaporust. It does get used up with usage. Phosphoric acid seems limitless, to the extent that I've used it anyway. A sludge will develop on the bottom of the tub, but just pour off the good stuff on top and reuse again and again.