If you lay Ospho on too thick, it layers up into a black coating that requires a lot of work to get it re-activated with fresh Ospho and then neutralized/washed away- so use it sparingly...
I've used very little of it on my restoration after watching that YouTube video last year. If I do use it, I neutralize it, and then wash and scuff sand everything before priming. I think that Ospho's instructions for use as a "primer" are probably sound if you aren't using it on a vehicle... just my .02
