If you were to do a search for the number of body/paint folk who no longer use PPG DPLF epoxy, the 'test' is indicative of it's reputation for failure...
Also any prep other than sanding was not indicated.
But that's not why I go with SPI epoxy first, it's because I found rust under the filler over metal on my truck, where the filler was STILL in tack and full adhesion.
