To fix the cupping you need to stretch the edges to match the stretch that has already occurred in the center of the strop. I would try some neatsfoot oil and a little hand rubbing. Hopefully over time you can stretch the edges enough to get rid of the cupping. To help with the process;

Quote Originally Posted by Lemur View Post
You need to get some type of stabilizer that will spread the load over the strops width.
I have started adding stiffeners to the bottom of my strops where I hold it. On some I have stitched heavy leather on each side. On the strop I recently made I took strips of ebony I had cut as scale blanks and used gorilla glue to attach them to the bottom end on each side. The pieces of wood are about 1/2"x1/8" thick x 3" (3" wide strop). Sanded them up real nice and there you have it. I makes for a good handle while stropping and assures that the tension will be spread evenly across the entire width of the strop. Any type of wood will work as long as it's thick enough so it doesn't break when you palm rub your strop, Popsicle sticks might be a bit light, unless you double them up. Be creative, you probably won't make things any worse.