So...I don't currently have the time or space to actually work on my build, just been stripping the parts rig, cleaning up what I intend to reuse, reading a lot, learning a little, and generally overthinking/overcomplicating my plan/to-do list when I finally get around to starting.
Most recently I've been obsessing about rust, and how I'm going to stop it from happening here in the salt belt. I still have many hours of research on different coating systems, prep etc. before I ultimately decide that I can't decide...but I'm constantly haunted by what's hiding INSIDE of the frame rails, and how to get a good clean base in there to start from.

I've tried to accomplish this in the past with wire tube brushes, shop-vacs with small hoses attached, flushing with pressure washers, blowing out with compressors, and finally coating with something along the lines of eastwoods internal frame coat when I've convinced myself I've got it all....but I never do. And all that careful prep and coating of the outside inevitably starts to bubble through as the rust makes its way from the inside out.

However, I just read another thread about a guy replacing a section of his frame rail, and in his pictures, he had a section of floor pan cut out giving clear access to the inside of the frame.
So that got me thinking...one of my to-do's is pull the interior and rust-proof the floor pan from the inside, so while I've got everything out, I could cut out the channel of sheet metal over top of the frame rail cavity, and REALLY take care of the internal rust. Then one could simply weld back some sheet metal, or some heavier square stock, channel etc. from the top to help with strength.

It would take some measuring and careful/slow cutting to trace the rail from the top, but is there any good reason to not do this? I've seen folks caution about cutting out whole sections of rail to replace, for fear of the unibody folding in on itself, but I can't imagine that floor section between the rails is contributing much structural integrity, especially length-wise along the frame.

So why shouldn't I do it? Or better yet, has anyone done it?

 

Opening Pandoras box bro. Mega job also. A better condition body would be tons easier, but you do you!

 

Pandoras box in terms of asking the question? Or in opening up the rails?
Unfortunately a better condition body would not be tons easier where I live, or at the very least, crazy expensive. Cars will rust out here in 5-10 years if not treated with a fluid-film type coating AT LEAST every two years, most who do it do it annually. So needless to say there aren't many 20+ year old vehicles around in drivable condition. I've only seen 4 xjs for sale locally in as long as I can remember, two of which I bought (one for parts...chassis is beyond repair) one which came in from a far off land, rust free, but at a price tag of $10K. and another currently for sale, not sure what the chassis rust condition is, but they're asking $7K, very little if anything in the way of modifications, so a lot more than I'm willing to spend for a base for a trail rig. And it's not that the body is in BAD condition, but there's rust in there, and it's only going to get worse if I don't deal with it properly, which is nearly impossible to do through the holes in the frame.

Besides, I don't mind the work, the project/hobby of building it is going to be just as satisfactory as the end product. And if I'm already stripping the interior bare, and will have to do something to treat the interior of the frame regardless, it's really only the added work of cutting out the channel of flooring then welding something back in. And I'd planned on frame stiffeners mostly for piece of mind of the integrity of the frame rails, so if I went this route I'd skip those as well.

I did find a post on another forum where someone contemplated the exact same thing, and was met with strong opinion, but no real reason it shouldn't be done. Ultimately it looks like he decided to just drill some holes from the top to spray in his chosen coating. I know there's more than just surface rust in there though, so no matter what I spray in, scale is going to come off (along with the coating), and I'm back to rust spots.

So lets say I open it up, clean out the rust, do an etch/por-15/prime/paint or other some other coating system, now I've got a zero point I can flush and oil/fluid film/rust check regularly.

What would give me the biggest benefit in terms of strengthing the rails from the inside? I'm thinking along the lines of a piece of square or rectangular stock fit tight into the rail. I'd put some seam sealer or other compound in first to seal the bottom edges so moisture couldn't get trapped in between, then weld in at the top. But because it would only be welded in at the top, would extending it down into the frame add any real strength? My gut says yes, as essentially that's what the frame rail is anyway, so you'd basically be putting a slightly shorter, stronger frame rail inside the sheet metal rail. You could also go a step further, and cut slits into the stock rail, to weld the inner rail onto the sides. (the same way the frame stiffeners weld on, but opposite). I'd just have to make sure it cleared any bolts that go into the existing frame.

 

I am in Michigan, rust is life, lol. You seem ready to work, the inner new material added would be on my list for sure. I actually made two heavy duty frame rails to mount under the unirail for new attachment points for new suspension. Frame stiffeners purchased, then decided to go for new rails. It's all good, take pics of your build, inspire some others 👍

The blue beast. Fabbed 1 ton steering, aluminum grill, many "me built" pieces on her. ( career machinist guy). Parts are free if ya make em! Ric