Use caulk to seal the area around tubs, sinks, and other wet area enclosures. It prevents water from seeping into areas where the moisture would eventually create mildew, mold, or rot the underlying structure. Several different types of caulk are used to seal the area around a tub. The best choice depends on the tub materials.
Silicone caulk is flexible. This makes it the ideal form of caulk to use under and around a fiberglass enclosure. Fiberglass moves when it fills with water or when someone steps into the tub or shower enclosure. This motion is slight, but if the caulk does not flex with the motion, it will eventually pull away from the enclosure, allowing moisture in.
For ceramic or cast iron tubs, flexibility is not a concern, because these materials do not flex. A caulk that contains a combination of acrylic and latex, or a PVA caulk works well in these situations. These types of caulks are rigid, but they have other advantages over silicone caulk. This, however, should not of concern when using them as a tub caulk.
Silicone caulk is difficult to clean up after application. Any residual caulk that ends up in an area where it is not needed, such as on adjacent flooring or your hands, requires aggressive cleaning with alcohol to remove. Silicone caulk also will not adhere to older, cured silicone caulk. This means that when working on a repair or remodeling job, it is necessary to remove the old caulk completely before the new caulk is applied. Silicone does have an advantage in that it is the longest lasting form of tub caulk, so any job, completed properly, should not need to be redone for a long time.
Silicone caulk has another advantage for use as a tub caulk. It is naturally mildew resistant. It is possible to purchase other types of tub caulk that have mildewcide added to the formulation, but the mildewcide is hazardous and not recommended in areas where it may be exposed to food or eating utensils, such as around the kitchen sink or dishwasher.
Acrylic latex caulk and PVA caulk are both general-purpose caulks that work well under most conditions. They cure quickly, are easy to use, and are easy to clean up. These caulks works best in areas where the caulk line is narrow. If the area has a thick area that needs filled in, silicone caulk is a better choice. Neither acrylic latex caulk, nor PVA caulk, naturally repel mildew, so be sure to choose one that has mildewcide added for use as a tub caulk.