Concrete walls are not like rock or stone. Concrete is porous and water can seep through the wall and can even be trapped inside walls themselves especially in basements or on the face of a retaining wall with earth behind it. This trapped moisture can accumulate inside the wall and harbor mold and mildew. This is why many basements have problems with mold and mildew and tend to smell musty. An average basement can let in more than 15 gallons of moisture in a single day right through the concrete walls and floor.
Sealing the walls and floors is the best way to prevent serious damage to furniture, carpets and household goods inside concrete walls. The sealer you choose should be made especially for concrete walls. Latex or acrylic products may cover the wall, but do not penetrate well, are pourous and cannot seal out water vapor that can seep through concrete walls. Concrete sealing products are made to wick up inside the pourous concrete surface and create a deep seal that goes into the concrete itself. To create a good deep seal, it is therefore important to prepare the surface of the wall or floor properly so the sealant can do its work well.
Other Moisture Proofing Methods
Plastic sheets are sometimes laid behind concrete walls or under concrete floors as a vapor barrier. Unfortunately, these are usually punctured during construction and can act as a trap for moisture between the concrete and the plastic. After several years, the lime in the concrete will break down the plastic and cause it to disintegrate. Plastic or insulation between concrete walls and drywall can create another moisture trap that should be avoided. Even painted on epoxy or urethane paints may hold back moisture for a while, but these also break down over time and allow trapped moisture, mold and mildew to seep through. Deep penetrating concrete sealer soaks into the pours of the concrete and reacts chemically with the lime and alkali in the concrete itself to expand, harden, bond and seal the concrete permanently.
Make sure the walls are clean and porous so they will soak up the seal. This means removing dust, dirt and debris with a wire brush. Clean oils, grease and any old paint off the wall. If you've removed old paint, etch the surface with muriatic acid (be sure and wear gloves and eye protection). Once you've cleaned the wall, wash down with clear water and allow to dry so that any volatile chemicals or acids are completely dried up. If the wall is new, it should be thoroughly cured before you seal it. Repair any cracks before sealing.
When and How to Seal
Concrete sealer often comes in a two part process. A primer is applied first and then the moisture/vapor barrier sealant, often in several coats. It's best to apply sealer in temperatures between 40 and 90 degrees. Apply in dry weather and allow two hours between coats. When finished, let it cure for 24 hours undisturbed. Sealing your walls properly will protect against moisture for the life of the wall. Seal any cracks that appear in the concrete itself promptly and you should have no problems.