In my experience, once rust has been completely taken off, one can control the surface rust as you go. John Willhoit (
http://www.willhoitautorestoration.com/ ) leaves cars bare for weeks at a time. They wash them continually with a light phosphoric acid solution while they do repairs and metal bumping.
Once the car is ready for paint, wash it, neutralize it, dry it and paint it with your favorite system. I don't think he uses any POR 15 because his cars are rust free when painted. The theory behind POR (paint over rust) is that rust needs O2 to be active. A polyurethane based paint, which I think POR formulas are like or similar, will work its way into the pores and seal off any remaining active rust from the atmosphere.
What I'm concluding is. that if you were to media blast a part, painting it with POR products would be overkill. Just painting it with formidable products would be quite enough. Marginal parts need protection and sealing from O2.
My .02, that's all.