One Long time Pro Shop owner tells me... The sand paper can harm, break down, or smooth partical in coverstock. Even wet sanding.
Another tells me that its ok.
Both are long time veterans of drilling, but I actually think sand paper will harm the coverstock. Makes sense to me.
So maybe scotchbrite pads are the safe way to go.
If anyone posseses any genuine knowledge on this subject, please reply. No speculations please.