Can you lay new ceramic tile over old ceramic tile in your bathroom if you sand it with 80 grit sand paper and put some kind of bonding agent or sealer on it?

If your old ceramic tile isn't cracking, loose or otherwise breaking apart then yes you can lay new tile over old. I wouldn't sand it with sand paper because I don't believe that sand paper will do enough scarcifying to the tile. get a right angle grinder, put a diamond blade used for tile on it and run it across the top of the tile. you don't need to take the whole surface off just have a few tracks running across each tile. Then you will need a more high performance thinset. whenever I have done this in for a client I use Laticrete 254 platinum or you can use a bonding agent. This is a roll on liquid that bonds to the existing tile and is a good substrate for the thinset you'll be using. You might have to go to a store that specializes in Tile to get these items. just as the sales rep there and they will point you in the right direction. hope this helps, good luck!