I am always on the lookout for inexpensive ceramic and terracotta pots to mosaic. Plastic doesn't work because the glue and tiles don't adhere well enough to withstand expansion and contraction and the seal isn't tight enough to withstand moisture. This is my experience. Other, more educated mosaic people may have found a way...
for this project I used a ceramic pot I found on sale at the end of the summer season. I didn't need to seal it because it was glazed. For terracotta pots, I brush on a concrete sealer to help reduce moisture transmission through the clay. This is also a free form sort of design. If I am doing a pattern, I will draw it on the pot before I start to add tile.
I buy my tile from lots of places. The tile on this project was from a tile outlet that has since closed. I get it at hardware stores, online sites and sometimes pre-cut from e-Bay. For this whole tile, I am using my smack with a hammer technique!
I lay the tile on a piece of cloth, cover the top and smack it with my hammer. (Lay the tile face down and the hammer will not damage the finished front of the tile.)
I use a lot of my used jars to store tile and cleaned styrofoam meat trays to hold the tiles which I am working with them. As you can also see, I use old tee shirts, dish towels, sponges and all kinds of stuff from around the house. The one thing I didn't manage to get a picture of is my nipper tool. I use that to cut the pieces I am using down to the size or shape that I need.
The red pieces I used were a pre-cut purchase.
When the tile has covered the surface, let it stand for a day to dry. Then tape off the areas which you don't want to grout. I have taped off the bottom pf the pot, but I want to get a good seal around the edge where the tile meets the pot at the bottom and top to help prevent moisture from getting up underneath.
I use grout from the hardware store. It comes in white, beige, black, and gray. I also use an acrylic admix to wet the grout. it makes it stronger and more flexible that using plain water. The grout should have enough liquid to make it spreadable but not drippy.
Oh, and use gloves for this, the grout is really tough on your hands! Some people spread the grout using a tool, I am more hands on and scoop the grout onto the piece and then smoosh it down into the cracks.
After the entire surface is covered and all the cracks are filled in, the surface can be lightly wiped down. After about 10 minutes, use a wet sponge to wipe the surface again, but not so much that it pulls the wet grout out from the cracks.
I use a dish tub with several inches of water in it for this process. You should not wash the grout down the drain in your house because it will dry and clog things up! By using the dish tub of water, I can wash the gloves off, and used the sponge to wipe the pot down without using the sink. After the grout has settled to the bottom of the tub, I can pour off the water and then wipe out the solids of the grout which remain and toss that in the trash.
After several hours you can wipe the surface again, What you are doing is taking the grout off the surface of the tiles, but leaving the cracks completely filled. The next day the tape can be removed and all the surfaces wiped and smoothed. I use a spray bottle of vinegar water to help loosen up any grout from the surfaces of the tiles and a rough cloth and a popsicle stick to scrape tough spots.
The last step is to seal the grout. I use a spray on sealer from the hardware store. Let it sit for a day and then fill your pot!