canvas mat

I made a canvas mat for in front of my sink area. 2'x3' I sealed it with a non-yellowing polyurathane doing about 10-15 coats. When I put it on the floor the solid grey background got scratched, even with all the coats of poyurathane. Do you have any ideas what I did wrong and a way to correct it. The only thing I can come up with is the background is solid dark grey. HELP