I'm not a body repair expert but FWIW I've used a few different brands of rust killers, all have Phosphoric acid, different countries will have different products/brands available. IMO they all work about the same, remove surface rust, apply the product for any rust remaining especially pittting that you can't remove, let it sit ?? minutes (depending on the product), then rinse off. Some require a repeat application. For a quick job like this about any aerosol primer then aerosal paint designed for autos/metal. Best would be an epoxy primer and a catalyzed paint (2K or two part) but then you're getting into more money.
The 3M cavity wax is just that, a light tan waxy liquid that goes on wet then the solvent flashes off leaving a slightly tacky wax. It can be used to help rustproof an entire vehicle and is used by body shops generally after a body panel is removed to bring that area back to OEM specs regarding rustproofing.
I use it for example while working on a door after cleaning it out, or a quarter panel, or tailgate cavity. For difficult to get area you can spray it in any small hole sometimes by removing a hole cover/plate, a bolt, or even a screw.
There is also a Wand kit of different length tubes/nozzles that can be purchased separately, comes with a 3 foot and 2 foot wand with tips that spray in multiple directions and a 8 inch nozzle. The longer wands help spray the wax deep into a body panel/cavity. The shorty can be used to flood seems and joints but you don't want to block any drain holes of course.
Some examples: I removed one screw for the latch thingy on the lower side of the upper hatch, then stuck the three foot long wand into that threaded hole and sprayed the product as I pulled the wand out. Did both sides and repeat. Idea was to coat the lower inside of the hatch where rust often occurs. The wax is thin enough where it shouldn't interfer with the lock linkage.
The upper aspect could be reached via where the washer nozzle hose or the wiring goes into the hatch. Or you could remove the lower plastic panel on the hatch, then open up the access panels and snake the wands in that way.
Same for the Tailgate; either remove a bolt for the latch mechanism and snake the wand in that way, or maybe better, remove the carpet then the access panels and spray directly.
For the rocker panels many ways to get into there; removing a rubber hole plug on the inner aspect of the rocker panel from either end would be easiest. Don't spray too much and plug the drain slits.
Hood and fenders, plenty of access.
Quarter panels: you could remove the rubber drain plug (which might tear), and spray in that way, or better, remove the panels in the cargo area. That allows you to check the condition of the bottom of the cavity to see if rust has already started. One problem there is to be careful how much you spray as you don't want the mist to come back out onto your carpet. Only the bottom and rear aspect of the cavity and the section that runs forward around the wheel hump and down to meet up with the rocker panel need to be coated IMO.
But for the current pinch weld project after the paint is cured I'll spray some directly on the pinch weld and then I'll probably swipe it with a cloth saturated in the wax to spread it around evenly Once the wax/solvent dries I'll install a new gasket.
