New cars aren't even undercoated... my 230k mile 2001 truck still has primer on the underide and it's driven year round, mud/gravel, snow/salt etc.
There's no need for either other than the wheel wells and inner quarters unless you want to cover up flaws.
This right here is just frigging hilarious. I've worked from Washington, PA to Philly and all over Ohio the past 20 years. I know what gravel Roads which is Limestone and Salt and Calcium Choride does to your car. I've watched 2 Dodge trucks a Ford and Chevy just Rot away. I've used Undercoating on all my hunting rigs in the past and I love the dirty gray look it leaves when you try and clean it up lol. If I had a trailer queen or live in Miami Beach or Southern Cal I wouldn't use either but I drive my cars. So after reading your hilarious comment I'm done with this post.
You seem to think you know more than you actually do... all the rams you see rust out in the wheel wells for example is because they trapped dirt/junk between the wheel house and bed side. Nothing undercoat or bedliner will change... goes the same for gm and fords. No under coat or bedliner is going to be able to get into this area. In fact it would just help trap it more than just pressure washing it now and then. Your not bedlining inside the frame, body mount areas etc. If you want to avoid rust out, your better off spraying oil on them.
230k and northeast 4x4 driven winter/gravel etc and yet my trucks not rusted out underneath... explain that keyboard pro! Go lay some more pipe, you obviously have no idea what your talking about.
