Every tile installation contains multiple joints and types of joints. For example, grout joints separate tiles installed on one surface, while expansion joints exist between changes in surfaces, such as where wall tile meets floor tile or where two walls meet in a corner. Expansion tile joints are always filled with a flexible, waterproof material such as silicone rather than a cement-based material like grout. Silicone must be tooled into the tile joints quickly to create a tight seal. Use one of three methods to get the perfect joint.
Applying the Silicone
Products like silicone fill and seal up tile joints, but they are also extremely sticky and difficult to tool. To help create a successful joint and a successful tooling job, apply the silicone to the joint sparingly using a caulking gun or tube. Too much silicone will attach to the surface of the tiles and will cure before you can finish tooling, which means you will need to scrape it off with a razor blade. Apply just as much silicone to the joint as will gently round out of it. You should be able to touch the silicone in the joint with your fingertip without having it push out of the joint when you do so.
Tool With Your Finger
Some builders and homeowners use silicone in small areas where other caulks have failed in the past. For these small jobs, sometimes the right thing to tool the silicone with is your finger. To keep the silicone from sticking to your finger, wipe or dip it in denatured alcohol. Place your fingertip at one end of the freshly laid joint and apply gentle pressure to pull the caulk along and smooth it into place. This method doesn't work well for large jobs because silicone is very sticky and dries in less than a minute, meaning you will not have time to clean and rewet your finger again and again.
If you plan on caulking multiple joints or areas, consider investing in a striking or caulking tool to tool the silicone into the joints. Striking or caulking tools are made of metal, with a smooth, bent end that pushes and moves the silicone into place. The silicone doesn't stick to the smooth metal surface, which means you can use it over a large area or multiple joints without having to stop and wipe it clean. Once the silicone is laid in the joint, place the rounded edge of the tool against the joint and pull it in one continuous motion across the joint to tool it.
If you're applying silicone to a large area but don't plan on doing so again, a plastic spoon will work just as well at tooling the silicone as a more expensive tool. The rounded backside of the top of the spoon is the perfect size and shape to tool the silicone. The silicone will not stick to the plastic, either, so you can do multiple joints without stopping to clean it. Apply the silicone to the joint, then place the top, back of the spoon against the joint and pull it along the joint until the silicone is smoothed into place.