Ospho is an orthophosphoric acid solution. The value of Ospho is that it converts iron oxides (rust) to iron phosphate which is pretty inert. However, unless neutralized the treated surface will remain acidic. There have been considerable number of adhesion failures with polyester, epoxy, epoxy primers (barrier coatings) over Ospho treated metal. These will resemble large blisters. Adhesion problems can be eliminated. Let the treated metal nearly dry, then do a complete wetting and scrubbing of the treated area with clear water, then sand overall with 80 grit
before applying any epoxy or epoxy primers. (See a discussion here:
How to neutralize Ospho | Southern Polyurethanes Forum (spiuserforum.com)
My view (after suffering disappointments) is don't use acid treatments/cleaners (PPG, Ospho, POR 15) if the metal is pretty clean - which sanding with 80 will usually accomplish. All these treatments are converters - they do nothing to prevent future rust.
Charles