I "undercoat/rustproof" my vehicles with
Amsoil HDMP and they still look new several winters later. A single can will easily cover everything under a JK or JKU. It sprays on like WD-40 and when dry, leaves a very thin wax-like finish that repels water and so far has held up to dozens upon dozens of underbody flushes.
How I apply it is simply spray it on and then hand-wipe it because it has a slight orange tint if you spray it too much in one spot. One thing it doesn't do is plug up any drainage holes. Something else the HDMP does is stop rust after it's formed; it just penetrates any rusty areas (i.e. ball joint areas, steering knuckles, trailer hitch receiver, nuts&bolts) and darkens them. The hitch on my Grand Cherokee still looks like new 2yrs later after a few salty Chicago winters. I've only had my JK this first winter, but a friends JKUR treated the same way with HDMP still looks almost like new underneath.
If there's a con to using the HDMP, all I can find so far is that the film it leaves has a bit of tackiness to it after it dries which can pick up dust to make gloss black frame paint look a lot less glossy. So, while my silver JK's black frame is protected and stays black, it is a dull black rather than shiny new, if you know what I mean. When I wash either of my Jeeps, the water beads off the treated undercarriage like water beading off a freshly waxed hood. Highly recommended, and it's cheap too.