Caulking is a relatively easy, inexpensive way to save money, energy and the environment. It also protects from water leakage and helps prevent mold growth. If you're caulking for the first time, it's a simple process of sealing gaps between any places air and/or water could escape, such as window and door frames and counter top edges. But if you already have caulking in place, you might need to remove it before you apply new.
Acrylic vs. Silicone Caulk
Most caulking around the home uses either acrylic or silicone caulk. The best type of caulk depends on what (and where) you are caulking. Acrylic caulk is easy to use, quick to clean up and provides a neat seal. It doesn't repel moisture, and works well for painting applications when you're filling gaps between walls, ceiling and woodwork trim. However, it can shrink and crack over time, creating tiny spaces for air, moisture and water to slip through. Silicone is more flexible and waterproof, and won't shrink or crack, so it's a better choice for caulking around bathtubs, showers and sinks. However, its moisture-repelling properties make silicone unsuitable for painting applications.
Removing Old Caulking
If your old caulk is silicone, it may be best to remove it. Nothing adheres to silicone (not even silicone itself). Use a silicone remover or razor blade scraper tool to carefully remove the old caulk. If you use silicone remover, follow the directions for use outlined on the container. If you use a razor tool, hold it at a low angle and push gently to cut into the caulk. If the caulk was recently applied and is acrylic, try softening it with isopropyl alcohol (but take care not to get alcohol onto your uncaulked surface).
When all old caulk is gone, thoroughly clean the area with a household cleaner, rubbing alcohol or wire brush, vacuum if necessary and wipe with a clean cloth. The area should be completely clean, dry and free of all traces of grease, dirt and chemical cleaner before applying the new caulk.
New Caulk Over Old Caulk
It's not always necessary to remove old caulk before applying new. You can apply a wider bead of new caulk, which covers the old bead and sticks to the uncaulked surfaces on both sides. Clean the surface of the bead and surrounding areas with rubbing alcohol to get rid of as much oil as possible. Test a small area with the wider bead of caulk to ensure it sticks well. (If oil is still lingering on the old caulk, new caulk won't stick.) Apply a wider bead of new caulk with a caulking gun, making sure it adheres to the surfaces on either side of the original caulk.