I havn't used their POR-20 before, but I have used their POR-15. The POR-15 requires you to follow the procedures to the T, and essentially nothing will stick to it after it has dried. Sorry, this info doesn't pertain to the POR-20 but I would ask a lot of questions to their techs to make sure you get it right the first time if it's anything like their POR-15.
Background, my dad had the ENTIRE body and frame of his Willys CJ painted with POR-15. Great plan, no rust....ever. After the POR-15 had been sprayed on, he found piss poor body work and other stuff wrong. Okay, we'll just fix it and go from there(the body shop went out of business for tax evasion) This stuff just won't come off, a week in a 30 gallon trash can of lye/water didn't touch it, it's a PITA to sand off, and well getting it right the first time is extremely extremely important with the POR-15. Then you can't paint POR-15 to itself once the first coat has dried, scuffing didn't help. We're now trying to figure out what will stick and be a decent base for his top coat.
It really is awesome stuff when you get it right, but if you don't, you're going to have a bad day. Or weeks, if you're my dad and can't paint your jeep because no amount of scuffing will make it stick to itself once dried.