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 therefor 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.
I am including our latest Technical Bulletin - please let me know if you need more info.
Stephen L. Pitcher
I read your bulliten. I just want to make sure that I understand one specific point. You say that once the OSHO has dried, typically overnight, there is no longer any acid on the surface to react with the paint. It is totally neutralized?
Yes, the Ospho will be totally neutralized by reaction with both the rust particles and the steel underneath. A quick water rinse after it has dried (remember, Ospho is water-based) will remove any trace of reaction particles (dust-like) and make sure the surface is in perfect condition for priming.
Stephen L. Pitcher
*courtesy of Hotrodders website