I've always used a brush, never sprayed it on, but I think my problem with POR was related to shelf life, which seems to be critical with these products. Some of their disbutors are mom & pop operations, and during the off season (Summertime here in Florida) there are few shows or swap meets - they don't sell a lot of the stuff and it sits around. As far as I know the cans aren't marked with an expiration date so the paint could have been fairly old when I bought it. I also had poor results with the silver POR on a set of monoleaf springs, but of a different nature - the contents just would not completely combine in the can, and resulted in a lot of streaks in the paint, like my springs had been "antiqued". This time, the distributor said it was probably an old batch, and that they'd "had some problems" with the silver and replaced it free. But it has held up fine for over 5 years, so whatever the problem was, it didn't act like the black.
One thing POR is great for is patching rust holes using fiberglass cloth. I had some rust-out spots in the cab floor and repaired them this way until I could afford new floorboards, and had to use a sawzall to cut through these areas when I finally fixed it right a few years later. Awesome chemistry in this stuff.
