IMO sandpaper is easier to use and more effective than plates, cups, or the top of a car window. I've tried em all.
I keep a small sheet of 800 grit folded up in my wallet for times I need to touch-up on the go.
You can lay it flat on a desk, wrap it around a pencil for a rod, or wrap it around a sharp lip on a countertop, book, desk etc. to get serrated edges sharpened. Versatile stuff.
Favorites: Pacific Salt, Police 4, Manix XL DLC, Swick 3, Aqua Salt
Looking forward to: FFG Pacific Salt, FRN Police 4, Swick 5, Native Chief