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There are two main types of caulk – silicone and acrylic latex. Silicone caulk is a completely waterproof and flexible sealant. It will bond with a lot of different kinds of surfaces. Silicone is a good choice if you’re caulking outdoors, or wet indoor spots like around shower stalls and bathtubs. It’s quite expensive and doesn’t do well in high-traffic areas.  Silicone caulk is ideal for areas of high moisture that need to remain mildew-free. Silicone caulk can also be useful when you need to adhere surfaces, like fastening countertops to walls or putting a top on a vanity. Silicone caulk is typically only available in three shades – clear, white, and almond. Silicone caulk cannot be painted over. Acrylic latex caulk is easier to use and cheaper than silicone. It can be used for a wider range of projects and it comes in a rainbow assortment of colors to match bathtub and sink glazes. Acrylic latex is water-based so it’s best used indoors. It is not as durable as silicone caulk and won’t last as long.  Since acrylic latex is water-based, it is possible to paint over it.  Acrylic latex caulk shrinks up more than silicone. It also dries much harder. Unless specifically noted on the packaging, latex caulk isn't suitable for any exterior surfaces. Acrylic latex caulk is not suitable for tiling or areas with direct sunlight. Old caulk must be removed before you add more. Use a utility knife, razor scraper or painter’s tool to cut away existing caulk. Use quick, sharp strokes. Keep in mind that metal blades aren’t ideal to use around plastic fixtures. Use a tool with a plastic razor blade, which you can get at any home improvement store, to avoid damaging the plastic.  For concrete and masonry applications, use a wire brush for removal. Home improvement stores sell caulk remover products that will chemically remove old caulk. Keep in mind that these products will damage plastic surfaces. For silicone caulk, you may just need to rip up enough that you can grab an end with a pair of needlenose pliers. Then, you can pull the caulk out in one long strip. Use a brush or vacuum to remove any remaining debris from the work area. Scour the surface clean with a dry, nonabrasive sponge or pad. Use rubbing alcohol to remove soap scum and body oils from a bathtub or shower stall. Dry the area completely before you begin caulking.  Both kinds of caulk need a clean surface, but if you’re using silicone, it’s imperative that you remove all debris and residue from the surface before you begin. For silicone clean up, soak a rag with mineral spirits (such as turpentine) and wipe the surface down with it. Then use a damp rag to wipe the surface down again. Apply strips of the tape to protect any surface area where caulk shouldn’t appear. Put them down vertically on either side of the area you plan to caulk. The painter’s tape acts as a guide to ensure your caulk lines are straight and uniform. The end result will look more polished and professional if you use painter's tape.

Summary:
Use silicone caulk if you need a neutral-colored waterproof sealant. Use acrylic latex caulk if you need a colored water-based sealant. Remove existing caulk from your work surface. Clean the surface. Use blue painter’s tape to mask off areas and ensure straight lines.