Silicone caulk is as easy to apply as acrylic or latex, but getting it off once it cures is another story. Solvents can soften it, but none will dissolve it, and because silicone caulk adheres so well to just about anything, it's almost impossible to pull off. Unfortunately, old beads that have degraded have to be removed before a new bead can be applied, because fresh caulk doesn't adhere well to caulk that has cured.
Softening With Solvents
When you're trying to remove silicone caulk from glass or tile, softening it with a solvent can make it easier to scrape off. Effective solvents include mineral spirits, isopropyl alcohol and white vinegar. Soaking a rag in one of these solvents and leaving it in contact with old caulk overnight will weaken its bond with the substrate somewhat, but not enough, in most cases, for you to pull all it off with your fingers. You may be able to remove some, but you'll still need a sharp implement, such as a knife or razor blade, to remove the rest.
Using a Knife
Whether or not you soften silicone caulk with a solvent, a sharp knife is usually an effective tool for getting it off. It works best on smooth, hard substrates like glass, ceramic, fiberglass or metal. The bead is easier to remove if you first cut it in half lengthwise. You can then work the knife behind each half, getting it as close to the substrate as possible. Once a small part has separated, you can pull it with your fingers while you continue cutting. If the surface is painted, some paint may pull away with the caulk.
Using a Handheld Oscillating Tool
A handheld oscillating tool fitted with a cutting accessory can remove silicone caulk more easily from porous surfaces like brick, concrete and wood than a knife. The blade acts like a rapidly vibrating scraper, penetrating the silicone bond and loosening it with little effort on your part. It is more aggressive than a knife, however, and is more likely to damage the substrate if not properly controlled. To prevent such damage, the blade should be kept as close to parallel as possible to the surface from which you're removing the caulk and not allowed to dig deeply into the corner. Don't use this tool to remove silicone from glass.
A Combination of Methods
When you're removing an old bead of silicone caulk, it's important to get all of it because it will interfere with the adhesion of paint or fresh caulk. A hard-to-remove bead may require a combination of methods that may include softening with a solvent and cutting with a knife and oscillating tool. The final bits may need to be scraped off with a pull scraper, because sandpaper is ineffective for removing it. Silicone caulk beads up when rubbed with sandpaper and clogs the paper. A final rubdown with mineral spirits should prepare the surface for recoating.