Recycling Rusty Fasteners
What is ospho? What does it do?
The primary active ingredient in Ospho is phosphoric acid, it works by "converting" iron oxide (rust) to iron phosphates that are much more chemically stable and therefore resist corrosion for a much longer time - thereby extending the effective lifespan of the primer and topcoat. After the Ospho reacts with the steel surface it is completely neutralized and does not leave behind any film or deposits that might interfere with subsequent primer/topcoats.
There can be a small amount of powdery-like material that should be blown, brushed or rinsed off before painting. It is also important to allow adequate drying time before painting. Ospho is a water-based solution and the water can hide in nooks and crannies causing adhesion failure of the primer. Old car frames are particularly susceptible to this because of the pitting commonly found on frame members.
Ospho does not have any known compatibility issues with any modern coatings (epoxies, polyurethanes, etc.) but as an old paint chemist I always recommend coating a small test area to make sure the entire coating system is going to work well together. I will also add the fact that the ultimate success or failure of any paint job on any substrate is about 95% dependent on the surface preparation done prior to picking up a brush.