I've gotten a lot of questions over the years about grouting. I understand why. My very first mosaic piece I did I left ungrouted and gave it away without ever grouting it. It's a skill not many people use these days and the first time it can be intimidating since you can easily ruin a piece with a bad grout job. I always recommend starting with something small to learn the technique.
The first thing you need to do is mosaic something. I had this old mirror (very old) IKEA mirror laying around in my studio. I started a mosaic on it a couple years ago, didn't like the way it was going and then abandoned it. Then, a couple of months ago, I got a chisel and scraped all the tile and adhesive off and started over. Sometimes that is just how a project goes: start, correct, rebuild, complete.
I knew this mosaic was never going outside so I just used 100% silicone for the adhesive. Wear rubber gloves and make sure your space is well ventilated if you use silicone. Nice thing is that it sets in just a couple hours.
Before you start mixing any grout, clean off the piece you are about to grout and scrape out any adhesive that has squeezed up between the tiles. It might seem like a negligible amount but if you are using a white adhesive (like I was) with black grout (like I was) it can show up pretty easily.
Tools needed for grouting:
*2 buckets--one full of cool water, one for mixing grout in
*Sanded Grout--very important that it is sanded. Unsanded is only for teeny, tiny gaps, around 1/16th of an inch--I have seen very few mosaics with this small of gaps. The sand is filler so that the grout won't shrink.
*Gloves. I usually wear a pair of heavy duty gloves over a pair of latex gloves.
*Sponge with rounded edges
*Knee pads handy if grouting on floor
NOTE:Some people might wonder where my grout float is. That's a tool used to push grout into the spaces. I just use my gloves hands. Usually the tile surface might be slightly uneven or I used tiles of different thicknesses and your hands are a much better tool.
Add cool water to your grout slowly. You don't need much grout for a small mosaic project. You can buy affordable sanded grout at Home Depot or Lowe's or most hardware stores. NEVER buy premixed grouts--that means that they are water soluble. That might be OK for an interior project, but sanded is stronger and easy to use once you get the hang of it.
This bucket of grout had over twice the amount I needed for the mirror. I was grouting a patch of tile in our kitchen simultaneously.
Make sure you are absolutely ready when you start to mix the grout. This is a cement-based product. You can't just add water to it in a half an hour if you get distracted or the phone rings. Once it starts setting, you can't reverse it. Once, I mixed a giant bucket of grout on a very warm summer day and then right as I was carrying it to the mosaic, my contact popped out of my eye and it took me 20 minutes to find the contact and then I had to go clean it and put it back in my eye and then over half an hour had passed and it was a big project and the grout was starting to set and I had to dump it and start over. Bummer.
Add water slowly. You will be surprised how little you need. Mix well; you want the grout to be the consistency of thick cake batter. No oozing, no puddles of water. The less water you use, the quicker the grouting will go and the stronger your piece will be.
Make sure your area is well protected before you start. And then just go for it.
Seriously. Just start dumping the grout on the mosaic and pushing it into the crevices. Move your hands from all angles so that you don't get any gaps. Take your time as this is really the most important part. You want the grout packed tightly in between the spaces.
Gently scrape off all the chunks that you can. Next is the hard part.
You have to let the grout sit for at least 10 minutes before the next stage.
This is where most grout failures occur. It's enticing, that grout. You just want to start cleaning it off immediately. But the key is letting it set up just a little bit. If you start wiping it off right away, the wet grout will be swept out of the gaps by the sponge and you will get a lot of shrinkage and a poorly grouted object. I like to clean out my grout bucket at this time. Save a little blob of grout on the side in case you missed a spot and need to patch.
NOTE: Do not forget that these are cement-based products. Do not pour any of it down the drain. Scrape out anything in the bucket directly into a garbage bag and rinse the bucket outside.
After 10 minutes or so has passed, dip your big sponge into the clean bucket of water and ring out all the water you can. You just want the sponge barely damp. Remember, water weakens your grout.
This stage I call the dirty wipe. Gently rub the sponge in circles across the mosaic. You are not trying to clean off the mosaic yet, you are just getting the grout to the level of the tiles and making sure it is well distributed. DO NOT CLEAN YOUR SPONGE. Let it get dirty. You are not cleaning yet.
This is a good time, however, to make sure your edges are well grouted and I cleaned out the inside edge next to the mirror at this point so there wouldn't be extreme grout buildup. Also, at this stage you usually will see if you missed any little gaps and any extra bit of grout you had sitting to the side can be used to patch.
Now let it sit for another 10 minutes or so. If your bucket of water seems dirty already, that is okay. It won't affect the grouting. In fact, even when you start to clean and your water gets black, it's OK, you don't need to waste time changing the water. (Unless you are grouting a whole kitchen, then you might change out the water after a bit.)
Now you are finally ready for the cleaning. If you have let the grout set the two times for 10 minutes each, this should go easily. Clean your sponge in the water and squeeze out all excess water. Then, carefully, slowly, and with firm pressure, wipe one area of your mosaic.
It should come fairly clean on one swipe.
Then turn your sponge over and wipe the next area.
Once you have wiped once with each side of the sponge, it is time to rinse it out, squeeze out all the water, and begin to clean another spot. This is the most tedious of the grouting process.
You have to clean the sponge between wipes. If you keep trying to clean off your mosaic with a dirty sponge, you won't get anywhere it life; you will be cleaning the same spot forever. Also, if the grout seems really wet, let it sit another 10 minutes before cleaning again. I grouted this mosaic last week in a studio that was only about 58F, so it took me an hour and a half. In warmer temperatures in the summer, I would be done in 45 minutes.
After you have finished grouting and your mosaic is looking pretty clean, do a wipe down with your shop towel. You mosaic will get a "grout haze" on it from trace water evaporating and leaving grout dust on the mosaic.
You can see below that I cleaned the mosaic, but it is still dusty. Then I did another clean with a dry towel with some pledge furniture polish on it; that oil soaks up the dust pronto. If you don't have any Pledge handy, you can use window cleaner. But spray it on the rag, not the mosaic itself and buff out the dust. You may have to clean it again the next day.
The cats are always so confused by mirrors, the pea brains.
The completed mirror. For this mirror I cut up stained glass (the gray) and used metal and glass tiles as accents. I grouted the piece with Custom Brand Grout in Charcoal. The grout will lighten significantly once it sets and the water is all evaporated. Always use a shade of grout darker than you think you need. I never use white grout. It is too glaring and takes away from the mosaic itself.
Good luck with your grouting!