: Just put the tiles on the mirror before you pour the plaster in. leave the connecting stuff on and put it away from the mirror. The tiles won't float up so don't worry. After you get the plaster set, tun it over and make up some thinner stuff and fill in any bubbles that you find if you want to. The other thing that you can do is to just use some thick paint to fill the holes in the plaster. The thing here is to fill places where larger grits can stay after you have cleaned the tool off. : The more I answer these questions, the more I am starting to feel like writing a procedure for making a mirror. : BTW, got a new guy in the workshop and he's got a 6" mirror almost rough ground in about 2 hours. Wont the plaster flow down to the surface ? I have tiles with a small blob of glue between them and I cant see that stopping the flow before set.