General question: I've seen many examples on here of how well, smooth, great, exemplary, etc. jobs that are done to finish a body panel and smooth it over so it looks like nothing blemished the part in the first place. But, here is my query... what is done to the back side of the panel to prevent rust, etc. from ruining the visible side? Single sheet fenders and most doors can be accessed from the inside to smooth/sand/paint the inside. But what about pillars and not so easy to access parts? Spray Por15 or Rustoleum the best you can and hope it covers?
Cavity wax can be used inside inaccessible areas. Capillary action will enable it to migrate throughout the part, helping to prevent rust or corrosion.
Tilt the body or part and pour the paint in. I keep an old mattress in my shop to lay bodies over on. Personally I’d never use a wax product because what happens if you need to paint the part again?
Sounds like the outside back room in an old apartment complex I used to live in...........I didn't like going back there, but the washer/dryer was back there. Man, I'm glad I don't live there anymore. Anyhow, I've used a undercoating gun that has a hose and a nozzle end. You drag it around as it sprays.
I coated the backside of all the panels on my Model A with POR 15. Some guys say they have problems with it not sticking, but everything I get it on is pretty permanent. That includes things I didn't want to get it on.
As stated by Double Pumper above, Cavity Wax is an Excellent way to protect the backside of any sheetmetal. VR&C.
As long as you don't get caught in the kitchen, I've used turkey basters with good success. Works well to seal door skins once they are installed. I would add that the separate parts were epoxy primed separately prior to ***embly. For tight access like inside of rockers, we've modified small paint rollers to fit inside those voids and provide coverage.
If you get good at that you would be a masterbaster. Just sayin. I use a shutz gun like posted above. They are dirt cheap and disposable. I sprayed raptorliner in the doors of my PU with one. Looked really nice but I had trouble getting sound deadening to stick to it. I used raptoliner on the underside of the cab after I replaced the floor pans, very nice finish if you don't want shiny paint down there.
While we're on the subject, getting coatings on panel-backsides isn't all that tough, I suppose, but what works - correctly - for getting in-be'twixt the crimped sheetmetal edges? Either OEM or replacement panels, both still rot eventually... I've noticed lots of rust-jacking there, esp along the bottom folded-over/crimped door edges, rockers, etc. I suppose they should be coated w/"something"(just what, exactly???) prior to the crimping being done, but when the deed's already done??? I've tried a couple things, like very liquid-y rust "converter(s)", & in the past, old (somewhat cleaned) used engine oil. Neither was really successful, but at least they were messy. Maybe I didn't do it often enough, nor used enough, idk, since they still rusted/rust-jacked/rotted. It is Minnesota & all it's wonderous salt & magnesium-chloride ensconced roads, after all, leading to those same chemicals invading anything/everything else. Solutions? Marcus...
I used this after I boxed my frame rails and some rocker panel rust repairs in an ot car. It was easy to work with and seemed to give good coverage. I can't comment on long term results as neither car sees very much weather. Phil https://www.eastwood.com/eastwood-internal-frame-coating-14oz-aerosol.html
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