I have experienced adhesion issues with POR in that it does not neccessarily play well with other products. I do find it useful for nooks and crannies that are difficult to properly prep (abrade) for typical paint adhesion. Given your cab has been sandblasted, I would be spraying epoxy primer, not POR. Then add your seam sealer within the recoat window, then spray with the top coat of your choice. Done..
