I was witness to undercoating back in the '60s and '70s. It never stopped rust.
It only delayed it to the eye. Underneath, moisture was doing its dirty deed.

Vehicles today don't really need undercoating. Just wash the underside after each exposure to salt.

See this thread:

https://www.jk-forum.com/forums/stock-jk-tech-12/maine-coast%3Drust-124367/

 

i bought my jeep brand new and im only 20. in 10-15 years i plan on restoring everything and makin it a badass rock climber. Even if i dont do that im sure having the frame and all under parts would help to be virtually rust free.

From what i have read Por 15 is the shit. Im thinking about using there kit to por 15 the whole frome and basically everything else under my jeep then going over it again with some rustoleum. any one think this would be good? is there anything in particular i should avoid spraying?

 

 

I've been using POR 15 on frames and bumpers for about 2 decades. It brushes on easily, just don't get it on yourself or it will have to wear off (learned this the hard way!).

For areas that get sunlight, a good rattle can paint in your choice of colors or black should be used to top coat it since it will oxidize in direct sunlight. For frames, I just brush it on and it's done.

It costs about $47 for a quart, you can order it through Quadratec and a few other places--and it's worth every penny!

 

I just check it once in a while and spot shoot the areas that are getting bad with some black rustoleum. It's been doing me good so far.