I use this tool along with cloth backed 320 grit sandpaper. The sand paper is folded lengthwise and put into the tool so there is grit on both sides. The two cover pieces hold the sandpaper in place. I secure the covers by inserting a piece of bent solid copper wire through the hole.

Link to sandpaper:

https://www.amazon.com/VSM-Abrasives-Aluminum-Weighted-Backing/dp/B07D5KSHF8