Paint Blistering

Hi there, Getting my butt kicked with a issue I have Never run Across. 2008 Chevy Pick-up Red 4x4 came in with rust issue patched the holes and repainted No big deal It came back 4 months later with blistering problems farther back than the repairs, Ok missed some, but when I removed the paint no holes what so ever but the metal was dis-colored looked like a weld spot but there was no welds there??? So  cleaned (sanded and wire brushed) the metal until it was uniform in color then put a skim coat of filler to even things up and primed and painted as usual, buffed it out put it together looking good go to pull it out and there's another blister where I know was repaired truck never left the building. Had a similar problem with a new 1955 Chevy door skin (Galvanized), Filler adhesion, but in 30 plus years this is new to me is there a issue with the factory galvanizing process? It acts like Teflon Coating.