craftgrrl | Hand-polishing rocks/resin

I am currently working on hand-polishing a pile of petosky stones, as well as some resin pendants. These are both done in the same way ~ where you wet sand them using several grades of sandpaper. (Technically I am using a dremel tool to sand the coarse and fine stages and sandpaper for the super-fine. The Dremel works wonderfully!) I know there are some girls who have done this on here, so I was wondering ~

1. How can you tell when you are done with one grade of sandpaper and should move onto the next? I don't seem to notice much difference between them. Will I not see the scratches until I add the polishing compound?

2. Where do you buy your polish? I would like to get some Serium Oxide (Cerium Oxide) but I don't think the guys at Home Depot had any idea what I was talking about. There was some polish for rock countertops that might work, but next time I will check in the glass section.

3. Resin question ~ I would like to dip some objects into the resin to coat them, but I can not think of how to let them dry. A few things have a loop (like for a pendant) that I can hang them by, but is there a way to coat other things? (Btw ~ after you sand your resin to remove the rough edges, you can dip it back into the resin for a smooth, clear coat instead of polishing it)