I've used a gallon or three of Ospho over the years and I'm not using it on any of my guns. I first used it on rusty holes in my 1978 Datsun B210. I've used it on old fuel oil tanks prior to painting, rusty mail box posts and other scaling, pitted pieces.
It's phosphoric acid and it's thinner than water - you can't control where it spreads. "a balanced formula of Phosphoric, Dichromate, Wetting Agents and Extenders "
I have part of a gallon left after recently scraping, treating and painting the iron railings on the front of my house. (After 100 years they needed resetting where they'd rusted off at the concrete steps and porch. The guy welded new heavy gauge legs on the railings and core drilled the old holes to reset them. Built me a gate, too.
) But I did the scraping, sanding, wire brushing, Ospho, primer and 2 top coats. The repair guy was right - spray paint. I asked about sandblasting it all while he had it down and at his shop, but he wasn't in favor of it.