It is important to remember to clean out the old caulk on and in the surfaces where you will be re-applying new caulk. Traces of dirt, paint, oil and old caulk left behind will only lead to problems later on. Use foam backer rod in joints that are 1/4" or wider to get better results and to save money. For those joints that are under heavy stress, such as in swimming pools, also think about using a primer.
Removal and Cleaning
Good surface and joint preparation is the real beginning of a professional and long-lasting caulking job, whether replacing old caulk or sealing a new joint for the first time. By using a putty knife, painter's 5-in-1 tool or other similar tool, remove all of the old caulk in the joint. A heat gun can be used to soften old caulk and loose paint to make removal easier, or a 3M Indoor/Outdoor caulk-remover can also help remove all types of old caulk. The surface needs to be completely free of old caulk, peeling paint, weathered wood fibers, grease, oil, wax, dirt, rust, frost, moisture, etc. A wire brush works well to remove contaminants, and a drill-mounted wire wheel is often the best answer for cleaning dirty, unsound concrete. To remove some contaminants like oil or grease, it may be necessary to wipe the joint down with a solvent-laden rag, letting the solvent completely evaporate before caulking. Remember, the best caulk in the world won't work if it is applied to a dirty or unsound surface.
Backer Rod: If the joint or crack to be sealed is 1/4" wide or wider, it is best to install a foam backer rod in the joint - to the proper depth - before applying the caulk.
Reasons to Use a Backer Rod: It saves money. A backer rod is generally cheaper than a good quality caulking compound, and most of the joint can be filled with the backer rod before the actual sealant is installed.
It provides the means to form an "hourglass" cross-sectional shape to the bead of sealant. This geometric shape allows the sealant to handle the inevitable joint movement much more easily than any other configuration. The reason: Large surface areas of adhesion are established at the sides of the joint, while a relatively thin cross-section of sealant is left in the center of the joint to allow for easy flexing.
It provides a "bond-breaking" surface at the rear of the joint or crack that prevents the sealant from establishing three-point adhesion - which, if allowed to occur, can lead to early failure.