I may have a solution to your restoration projects. I'm the guy that Mike Nickel referred to.I have been printing sides for Marx box cars and passenger cars. These are all redone artwork, not just a scan of an existing car. I will try to post photos of some of the sides as soon as I figure out how to upload them. Bear with me. I'm anxious to share my work.
I have sides for both 7" and scale boxcars. I started out making matching head end cars for the stock NYC passenger cars. I have also added a diner. Passenger sides also include a set op PRR maroon sides. I want to modify the Meteor car to PRR obs. Still thinking on this one.
These sides are printed on heavyweight archival photo paper with archival photo inks. I have several that I printed eight or nine years ago and have experienced no fading or other degradation of the images. That's why I have always used the best ink and paper.
Watch for the photos soon.
Joe