aj2494

Not if you cover it with a UV coat. Full disclosure - I haven't used POR-15 myself but I have done a ton of research into using it. POR-15 has many different steps, and like almido said, you really need to use all of them. First goes the degreaser, then the metal etching primer, then the POR, followed by a UV coat if the area will have direct light on it. The UV coat can be a clear layer or a paint that has UV protection built in, which wouldn't be necessary underneath a car. This is honestly the same process that should be followed for every bedliner or rust-proofing application. There's a reason our paint at the factory has a lot of steps and modern paint has quite a few more than that which is on our XJ's. Take the time and do it right

JeepJess

I used the NAPA rubberized undercoating for my wheel wells and it's awful. Didn't even last one winter here in CO. Given I pressure wash the MagCl off, but I expected a little more. I have a can of Por15 now, and going to try that on the chassis, wheel wells, and inside the tub (I pulled that carpet out and replaced the frt. pass. floor pan.) Hopefully it holds up better and keeps the rust away.