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Before applying new caulk, all existing caulk must be thoroughly removed. Begin by manually pulling off any large, already detached sections of old caulk. For caulk still adhered to the surface, you can score it with a utility knife to break its seal. A sharp painter's tool can then be used to scrape and break up the connection between the caulk and the surface. For areas that are difficult to clean, a specialized caulk remover tool can be effective, especially on hard surfaces like ceramic or porcelain tile. If removing caulk from softer surfaces like acrylic tubs, be extremely careful not to scratch or damage them; a plastic caulk remover tool might be safer. For stubborn residue, you can use isopropyl alcohol (91% or higher) applied with a spray bottle and wiped with a sponge, allowing it to sit for a few seconds first. Vinegar is another common household substance that can help loosen caulk. Some specialized chemical caulk removers like 'Goo Gone Caulk Remover' are also available, but carefully read their labels as some can damage acrylic surfaces. WD-40 is not recommended as it leaves a lubricant residue that prevents new caulk from adhering properly. A heat gun can also be used to soften caulk, making it easier to scrape, but use caution to avoid heat damage to surrounding materials.