I did my suspension and rear differential with por15. Holding up quite well after a winter of sand and salt. I didn't top coat it with anything. The thing about por15 is that it doesn't have any UV protection so any parts in direct sunlight will change from gloss black to flat black. That is why you have to top coat parts exposed to sun.
2008 WK "S" Limited bright metallic silver exterior, light graystone interior, 5 speed W5J400 transmission, June 18/2008 build, C213RE rear differential with 3.73 axle ratio, C200FE front differential with 3.73 axle ratio, 245/60R18 Goodyear Fortera tires (stock summer tires), NV245 transfer case, GT2056V turbo, 3.0L CRD (OM642.980)