Caulk is designed to stop water and is often used around sinks, showers and tubs. When you apply caulk you use a caulking gun that allows you to squeeze the caulk from the tube that it is packaged in. When the caulk is forced from the tube it will come out in a small thin bead.
You are supposed to direct that bead so that it lies perfectly against the surface you want to seal shut. At this point, you can use a caulk finishing tool that will press the caulk into any crack or crevice that is present and spread the material out so that it adheres to both sides of the surface you are protecting.
The finishing tool that you get for your caulk work may have a flat edge that resembles a tongue or it may have a head that resembles a large arrowhead. Both of these styles work great, but the arrowhead-shaped tool is better for getting caulk into tiny crevices, and it allows you to use the point of the tool to apply a greater amount of pressure and force the caulking material into fine cracks and crevices.
The handles of the tools may be wooden, plastic, or rubberized. These tools have to be flexible and capable of bending so they can contort and configure the caulking material into the shape it needs to be.
Once you have laid the bead of caulk you drag the caulking tool along the line of the bead, applying steady and even pressure so the caulk spreads and touches both sides of the joint or crack that you are trying to seal.
Once the caulking material is in place you must let it cure or dry the appropriate amount of time before you get any water on it or before you use the item you placed it around.
Things to use when you do not have a caulking finishing tool on hand
You can use your finger as a finishing tool. Most people dampen their fingers with water and then run them along the center of the bead of caulk, applying pressure so the caulk is flattened and smoothed at the same time.
This procedure only works with water-based caulks and while you are smoothing the material you must keep your finger wiped so that an abundance of caulk does not build up on your finger. You also have to keep your finger moistened or the caulk will start to stick to your skin and you will pull the bead up instead of flattening it and smoothing it out.
Finishing tools create a smoother and more uniform caulk bead.
You can moisten a cloth with the solvents that the caulk manufacturer recommends and cover your finger with the rag as you slide it down the bead of caulk. It is best if you wear a protective glove over your fingers when you are using a rag soaked in any solvent. Some solvents can dry your skin and cause irritation.
People often use plastic spoons to help them make water-based caulk lay down smoothly. The spoons are harder to use than the finishing tool because they are not as flexible, and they are not crafted for this purpose so only a portion of the spoon presses against the bead. The result is not as pretty as the caulk that a finishing tool is used on.
Some people use the handle of an old toothbrush to try and smooth out caulk around sinks and tubs. The real problem with this is the toothbrush is not flexible, and it tends to push too much of the caulk into the crack and you are left with a caulk line that has an indentation.