Hey there. I learned a nice little tricky for getting those confounded card frays out (you know like leaves of a book) and make them rigid and whole again.
You can use any kind of glue stick, but I would strongly suggest that you use an “Avery” brand glue stick. This brand of glue stick is acid free, it should say so on its card. It costs more than other glue sticks, but its worth it!
The stuff comes in a twist up tube and its pink but it dries clear, so there’s no worries about color tinging on the edge of the cardboard paper.
Get your frayed areas and examine them. after you’ve counted your layers, get yourself a thin toothpick and gently run it in between the frays, not hard, just enough to seperate them. When that is done, lightly scrape the top of your gluestick and lightly coat your frays top and bottom. You don’t have to butter them like toast, just get a “dab” (as we southerners say, y’all) on each top and bottom of fray.
Once you’ve done that, gently press the frays together. You may have some over gush coome out of the side, don’t worry about that, just scrape it off with your finger. Press down hard with your fingers for a minute to allow your corner to meld.
If some glue gets on your card, don’t panic! The glue Avery uses for its glue stick is very forgiving! Its open for about 3 minutes or so before it starts to harden. Slightly dampen a paper towel and gently remove the glue, its water soluable while its open so getting it off won’t be hard or require scrubbing.
After you are pretty sure there’s no glue where you don’t want it…find a flat surface and something flat and heavy to put on your card corner. You mat want to put some wax paper in between both the hard surface and your weight in respects to the card so if any glue you missed is still on it. The glue won’t adhere to the wax paper.
Let it sit overnight, in most cases if ther is no ridging or wear on the black/purple parts of the card it won’t eve look as if the card has been frayed at all. If nothing else it prevents further card corner degredation.
I’ve done this with Thundercats and Transformers boxes and had spectacular results.