Most folks don't shop for caulks and sealants like they do for bathroom fixtures, but considering the job that caulks are expected to do and their high visibility, it might not be a bad idea. The problem is, there are an awful lot of caulks and sealants on the market, so choosing among them is difficult. Caulks basically do two jobs in the bathroom: seal against moisture intrusion and provide a pleasing joint between fixtures and wall finish materials. For the most part, careful detailing will minimize the reliance upon caulk for both functions, but there are still instances when it is necessary.
Types of caulk
While there are about a dozen types of caulks available for residential use, caulks for use in bathrooms fall into three basic categories: latex, acrylic latex (sometimes with silicone), and silicone.
Latex caulks are easy to apply and easy to clean up because they're water based and hold paint well. I like to use them when painting with latex paint because they are cheap and fill cracks and holes easily, and can be painted over almost immediately. However, they aren't very water resistant or flexible, so they're a poor choice for general-purpose applications in the bathroom.
Acrylic latex caulks are more flexible than regular latex and are usually available in a fungicide-treated version for bathroom use. The fungicide gradually leaches out of the caulk over the course of about 5 to 15 years, helping to prevent mildew growth for that period of time. These caulks are a bit more expensive than plain latex caulk, but they are paintable and work well as a general-purpose caulk, which makes them worth the extra money. There are also siliconized versions of acrylic latex caulks, but the percentage of silicone is so low (typically less than 2%) that the caulk's performance is not appreciably altered. Most caulks that are tinted to match stock colors of different manufacturers fall into this category.
Silicone caulks in tub-and-tile versions that contain a fungicide are available, and though they cost considerably more than acrylic latex caulks, their durability and flexibility make them good performers in the bathroom environment. They do have some drawbacks, however, including the fact that they are difficult to work with: They set up fast, need a well-cleaned substrate to stick to, and are hard to form into a smooth bead. Silicone caulks aren't generally paintable either (even the so-called “paintable” ones), though the clear and white formulations cover most situations that you'll encounter in the bathroom. I've also noticed that some silicone caulks tend to get dirty easily, and when they do get dirty they are hard to get clean again.
Working with caulk
An open tube of caulk is a bit like Pandora's box, and it's hard to keep the mischief contained in it from spreading everywhere once it's opened. Part of the problem is that caulking is often approached as almost an afterthought. But a few simple steps will make caulking less of an annoyance and improve its appearance and performance.
1. Prepare the surface. Silicone caulk especially doesn't adhere well to dirty or contaminated surfaces, whether they are new or old. Old caulk should be removed from tubs and sinks, and all surfaces should be thoroughly cleaned of old soap film and dirt before recaulking. In severe cases, this may mean cleaning with a detergent, which should then be cleaned off with a water-soluble solvent, such as isopropanol, and allowed to dry. Rubbing alcohol also works well on soap film.
2. Prepare the caulk. Caulk should be worked at around room temperature, so cold tubes should be warmed up before using them. Different-size joints require different-size tip openings, but in general the smaller the tip opening the better. Many caulking guns have an integral nipper for cutting off the tip, but a sharp utility knife or shears do a better job because they are more accurate and leave a cleaner cut. A 45°angle cut allows the tip to be held against the joint without scraping out caulk, but a straight cut works well too, depending on the type and size of joint being caulked.
3. Tool the joint. I've pushed caulk in front of the tip, and I've pulled caulk; in some cases, you don't have a choice. In either case, the idea is to avoid leaving voids and to inject enough caulk into the joint. Outlining the joint with masking tape makes it easier to clean afterward and guarantees straight joint lines. Immediately after the caulk is applied, it will need to be tooled, which will help improve adhesion, remove air pockets, and smooth the joint surface. Special caulking finishers, plastic spoons, and even tongue depressors work better than fingertips for tooling the caulk and leave a smoother and moreprofessional-looking finish. And having a couple of rags handy to wipe up excess caulk from hands and tools will help to keep it under control.
4. Clean up. If you've used masking tape to outline the joints, be sure to remove it before the caulk begins to skin over. Most caulks indicate on their labels the appropriate solvent for cleanup.