"Natural" weathering: You are SO right, TrainFam!
Surprisingly, I have yet to see a Marx 3/16" boxcar that was too worn for my use. I like 'em all.
As for the wheels:
When finished running the "Piney Turn" on my wienie train layout, I sat down at the workbench and used my Dremel w/a wire brush and polished the wheel treads/flanges of my newly arrived NKP 71499 gondola. That's about the only extent of "rust" concerns I have (assuming the axles are "good 'enuf"). Here's a before/after pic:
Any other appearances of rust is one of those "no big deal" kind of things. Besides, nicks, dings, dents, grime and rust sort of gives each piece it's own "character", so to speak. It's all good!
Tempted to bid on a Marx 3/16" item or two I've found, but I'm also gettin' a hankerin' for a Lionel Baldwin boiler Hudson w/whistle tender, too. Decisions, decisions!
All fer now!
Andre