This to me seems like the way to go. Thanks for the product info. I was wondering how to deal with inside the frame. Is the product you used like a foam filler with anti rusting properties or just a coating. I can’t see how to fill the whole frame tho

The frame shouldn't be "filled". Don't block the ability of thhe inner frame to ventilate and dry.

I figured that being I saw what moisture getting behind the rubber undercoat can do

Not to "date" myself, but I first started hearing about undercoating trapping water against the metal in the 1960's. This is why it's important to really clean the metal before applying anything - and one should maintain the surface each year to be sure the protectant is still holding tightly.

To the OP. What do you want? Do you want it to look nice or do you want to stop the rust because in my experience the two don't go together unless you strip the entire jeep down and do a ground up restoration.
Yes Rust Converter (which has been around since the late 1940's) does work but it requires a top coat of paint within a week or so of it drying. If you miss a spot then that will deteriorate quickly. It is simply a mild acid. My 2008 looks worse than the 2015 Jeep in post #48. What I will do soon is get (what I believe to be) the best US invention for halting rust and making the vehicle last. That is a product called Rust Check. Shops everywhere apply it and in Canada we pay about $300 for first application, then yearly about $80-$100. It holds what you have at that level for many many years. They drill holes in the doors and all the inaccessible body panels and spray in there with a multi-directional spray head then install the caps which they remove in the next application. They also do the chassis, diffs etc and it does not harm anything.
I had a Dodge 3500 new in 2007 and sold it to a Washington state dealer in 2017 and he could not fault the truck with 10 years in a mild salty environment for winter roads. That truck had been rust checked from new. Look at Dodges 10 year old now in east Canada and East USA - they are rust buckets now.
Rust Check oozes out of every seam in a vehicle for about 2 weeks after applying but it displaces oxidation which causes rust.
Hope this is useful to you/.