I have used POR15 on a lot lately. I painted my "new" core support last year, front and rear axles, leaf springs, RSK, adjustable track bar, both new fuel tanks, fuel tank skid plates, fuel filler necks, etc.
I love it. Granted it makes installation of those parts take longer, but I feel the prep work to prevent rust is worth it.
On the core support, and axles, I had everything sandblasted then used the POR15 Metal Ready, following all of their directions before painting. On the RSK and other parts, I did not sandblast but instead stripped everything of grime or grease via Dawn soap, then used the POR15 Metal ready, then paint.
On the parts that would see moderate sun exposure (hubs, RSK,) I did top coat the POR15 with the POR15 Top Coat, which is UV resistant. Should prevent the paint turning purple, fading, and weakening.