In both its liquid or vapor form, water can cause cracking and change the alkaline nature of masonry materials. This will drastically weaken the hard material.
Water damage is very hard to fix once it has happened. Using an appropriate sealer can prevent much of this damage before it occurs and is much cheaper than the repairs after the fact.
Water damage can shows up as white chalky areas or streaks. This is called efflorescence and is deposits of calcium salts that are an important ingredient of stucco, brick, concrete block (cinder blocks) and concrete. Water dissolves these salts and transports them to the surface. If the surface is painted, blistering and eventual peeling will be evident as well. These areas can be soft and easily scraped.
The main types of sealers available are clear penetrating coatings, pigmented coatings and film forming products. All are available in either water base or oil base formulations. Painters favor the acrylic products because the formulations allow water vapor to escape and stop liquid water from entering the surface. Plus they are easier to apply.
Clear sealers can deepen or enrich the color of the surface, but usually will not be visible. These are water sealers and are designed to repel liquid water, but they are not waterproofers. Water vapor will still move through the material.
Water sealers stop liquid water from entering the surface, but allow water vapor out. They are breathable products that can be applied to brick, concrete block, stucco or any masonry material such as stone. Some are designed for use on floors, making them very useful for sealing decorative concrete stains.
Another type of clear masonry sealer is designed to combat graffiti. In this case the sealer is applied in several coats, typically 2-3. The final coat is a sacrificial coat that is pressure washed off with the graffiti. Another coat is applied after the surface has dried. This sealer forms a clear unnoticeable film.