I use the Iron Armor truck bed coating from Harbor Freight on the underside of my jeep. It's cheap, comes in an aerosol can, and is way better than the rustoleum bed liner, which seems to be mostly solvent. I reapply once per year (after power washing with Simple Green). The Iron Armor is PVC based, so It will degrade in UV light, which i why i only use it on the underbody. Jeep is nearly 5 years old, daily driven 4 of those years in the salty north winters, and still looks like new underneath.
For bumpers, roof racks, etc. in direct sunlight, I use multiple coats of Herculiner. It's polyurethane based, so it's immune to UV degradation, and unlike Linex, you can apply it yourself.
If i had the money, i would go linex all the way. Powder coating only lasts me about three years before rust starts.
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