Originally Posted by Esprit Aviation
I The other two will leave a thin phosphate coating which will protect the metal from subsequent rust, but only for a short time. The coating they leave may, or may not be acceptable for finish coating.
When using any of the Phosphoric acid rust removers such as Rust-Mort, Ospho, etc (when used a remover and not a "converter") a person can choose to leave the Phosphate coating or not. If it is left to dry then the coating will remain but if it is thoroughly rinsed with water before it dries it will leave clean metal. If it dries and the coating needs to be removed later then it is a simple matter of re-wetting with the product and then rinsing before it dries again.