Re: Paper water shields
Not to be contrarian, but here is my experience with the water shields behind the door and rear window panels: I've never had them.
I bought a 1972 Cutlass in 1992 and did a pragmatic restoration on it (got it running and working). I had the door panels on and off several times and there never were any water shields. The panels were in excellent shape. I drove that car as my daily for more than a decade, coast to coast, border to border, in the rain and snow, and the only time it was in a garage was for a few months during a Chicago winter. Other than that, it was driven regularly in rain, including several downpours, and when parked it sat in that rain. I had the door panels off once about 8 or 9 years after first buying it to adjust the windows. Still no water shields, and the insides of the doors were rust free and the door panels completely undamaged. It's not that I was trying to save money, it's just that the car didn't have water shields when I bought it and took it apart, so I didn't speculate that it needed them. I didn't know, and this was far before Internet sites like this one. After I sold the car to a family member, I helped with a second restoration and the doors were still free of rust and the interior door panels straight and intact.
I now have a 1969 Chevelle as my daily that I bought in 2006. I've had the door panels off twice, once a couple years after I bought it to adjust window fit, and again about eight years later when trying to minimize window rattle. This car also came without water shields, and while I do now have it garaged, I live in Oregon and it's seen its fair share of rain and downpours. The doors are still rust free and the interior panels still look like new. I did have the back interior panels off last November and the insides looked fine, and, as the front, the interior panels still look like new.
Taking my experiences with two cars over two decades over several thousands of miles and varying climates, and comparing them to other reports, those water shields may instead attract moisture, retain or trap it, and cause damage to panels they're supposed to protect. Maybe, maybe not. That's just a hindsight speculation, one that would be difficult to prove or disprove without controlled factors.
When I sold the Cutlass, it was run down but free of rust. I saw it a year later after it had been under a tarp and there was surface rust starting all over -- on the convertible rails, the air cleaner, the trunk, etc. I have a theory that even though I drove (and drive) these cars in all kinds of weather, I never let them sit in that weather for more than a little while. I believe that driving them often "airs" them out and prevents a lot of water damage and rust. For a hobby car that gets driven every once in while, washed, and sits for the rest of the time, maybe water shields are a good idea. I don't know. Take it for what it's worth.