FWIW removed the front and rear inner-facing rubber plugs and cleaned out the rocker panel cavities with car wash soap, then rinsed with water and let them dry.
IMO there appears to have been a very thin layer of some type of cavity wax (anti-corrosion) sprayed into the doors, inner body panels, rocker panels, tail-gate, etc when these vehicles were original manufactured. If you look (or take photos and feel) closely some internal areas that have never been touched you'll find traces of that wax IME.
I've posted this up before but for those who don't know about this product, I use 3M Cavity Wax for rust-proofing inside body cavities. Same stuff body shops use on new vehicles to put them back to manufacturers specs.
Once the body cavity is fully dried out (I've sometimes poured in 91% isopropyl alcohol to speed up the process) I then spray in the Cavity wax being careful not to plug any of the slit drains. The wand kit works well.
Scroll down and watch the video in this link:
Application tips: the video shows the area being flooded with spray. I've found if you apply too much product at the same time it may slump and leave a bare area on a vertical surface.
So what I've done is spray it maybe twice withdrawing the wand quickly then let the solvent flash off overnight, then go back the next day and repeat that process. I've also noticed that the wand tip does not spray equally 360' as it appears in the video so IMO it helps to rotate the wand a few degrees (ballpark 20-40) between applications so you get more even coverage with multiple light coats.
I'll do a test spray in the air before I start to check out the spray pattern. If the nozzle appears clogged it can be cleaned with something like TCE (Trichloroethyene) brake cleaner.
After spraying I go back with plastic zip ties to make sure I haven't blocked any slit drains.
There are also drain holes located at the bottom of the doors, easy to see, but there are a few slit drains in the bottom of the quarter panel cavities that are about impossible to see without a bright light and taking some photos.
FWIW one method I use to find those drains, often plugged with dirt, is to pour Isopropyl alcohol into the cavity then get under the vehicle and look for where it's wet or dripping. If extremely dirty it takes some patience to find the drains as some of them are just very small open spaces between body panels. Different size plastic zip ties can be used to help open up any plugged drain holes/slits. If anyone's curious, the reason I use the rubbing alcohol is so if it gets trapped anywhere it will just evaporate.
FWIW