The property that makes POR-15 superior is rust will not undercut the paints' grip on iron, there is no blistering surrounding a knick in a painted surface when using POR-15 .
My 1973 Airstream OEM undercoating was mostly intact inside the ladder frame rails, except at front and rear; everything outside of frame needed rehab though. The original frame paint loaded up anything I tried to use to remove it - and once removed left oily residue persisted after three or more solvent wipes so I rustoleumed those areas... With floor on & belly off I stretched one quart of PO-15 to put three coats where exposed to weather or meant for aluminum contact, and two coats on everything else. I also got their thinner and kept brush core damp with thinnner for smoother application & thinned the last 1/3 of can when it started gelling up from exposure to air.
I am one saw cut from pulling rear bath floor from my AS and want to treat the newly exposed iron, so it looks like I will have to try and use up a second quart
meaning everything I painted needs to be surface prepped for the next coat to bond. 3M Corp pancake abrasive discs on a die-grinder worked very well to remove crud before I painted so I will try them to scuff up POR-15 to recoat...