CAULK:

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The best siliconized latexes are a good choice for heavy-duty work, such as exterior windows and doors, and caulking seams in kitchens and bathrooms, and to keep moisture out of walls and floors.

SILICONE

Silicone sealants were first used to bond glass panels to skyscrapers - afar more demanding job than most home improvement projects. Because they stay flexible at all temperatures, are completely waterproof, bond well to almost everything and won't support mildew growth, silicones are used around sinks, tubs and shower stalls.

Silicones come in two types: neutral cure or acid cure. Acid-cure silicones work best on nonporous surfaces such as glass and glazed tile, but they can corrode metal and etch some plastics. Neutral-cure silicones work well on metal and wood .

Silicones aren't perfect. For starters, these caulks are hard to smooth, and most won't hold paint (one exception is GE's new XST paintable silicone). If you need to recaulk, the old residue is almost impossible to remove.

POLYURETHANE

Polyurethanes excel as outdoor caulks. Since they are non-corrosive, extremely tear resistant and stick reliably to almost anything, they're a good choice for joints between dissimilar materials, such as metal-to-masonry joints around chimneys, wood-to-concrete joints at the sill plate, and masonry joints in driveways and concrete slabs.

Polyurethanes are not naturally ultraviolet resistant, so exterior joints should be painted or otherwise protected from sunlight. Tooling joints isn't as easy as with latex caulks, but easier than with silicones.

The biggest downside to this type is price: Polyurethanes are more expensive than most other caulks. Use them where strength, durability and weatherproofing are most important.

RUBBER

These products are made with synthetic rubber compounds such as isoprene, butadiene, nitrile and styrene. Rubber caulks will also stick to almost everything (but they melt styrofoam) and will even work with damp and oily materials. Their biggest downside is their smell; the solvents used are highly flammable and dangerous to breathe. For that reason, this caulk should only be used outdoors. Another drawback is shrinkage: After the solvent evaporates, the bead can shrink by as much as 35 percent.

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