OSPHO is Phosphoric acid. Guess what's in naval jelly? The stuff is not that bad. Wear gloves, work outside, use a paint brush.

Frankly you would be better off finding someone that does Media Blasting and have the thing stripped completely. For example there's a guy around here that will strip a car frame and epoxy prime it for three hundred dollars. Way better than anything you can do and will last much longer. Look around maybe it'll be a better way to go.