Use this chip, attach a USB port on one end ... First, it makes a mess. This is wet/dry sandpaper, though, and you can and should sand with water. Secondly, even pressure should be applied.

Let it continue to dry. Sand the surface using 60-grit sandpaper. Wipe with a damp cloth or clean with a wet/dry vac. Repeat Steps 2 and 3 four times. On the 4th sanding, use 220-grit sandpaper.