Good info! :top:I've used Ospho on rust before. If the barrel is all covered with surface rust Ospho will turn any red rust into a black inert substance. On the bare metal or blued parts of the barrel it will leave a frosted like residue after it has dried. You will have to wire brush off this residue before painting the barrel. If you sand or wire brush the barrel you cannot brush off the inert back substance where the rust used to be or it will rust again. Don't breath in the frosted white residue when you are sanding or wire brushing the metal---stuff is bad for the health. Wear eye protection so you don't splash any in you eyes.
Ospho is phospheric acid---it will burn bare concrete just like battery acid does if you drop it on concrete.