Hi, I was just reading your column and it sparked a few questions about my house. I have an older home in the late 40's with sand coloured (light brown) stucco that has never been painted, and I believe that moisture (from rain) is getting thru to the inside. On two sides there are black marks running down the wall, like a nail or piece of wire was rotting and making the building cry black, if you will.
In order to stop this I believe I would have to paint the whole house, first by applying a sealer, and then the main coat. Am I correct and what sealer would you recommend?
Stucco, especially on older homes, is a very durable and cost effective siding material that performs well in our extreme climate. When originally installed this material may be left a natural colour, like your home, or tinted to almost any colour desired. The only drawback is that the appearance may deteriorate over time with hairline cracks and other cosmetic issues. Painting the stucco will improve the appearance considerably, but once the stucco is painted, it will have to re-painted on a regular basis.
The causes of the black stains in your stucco should be further investigated before proceeding with any painting. The black stains may indeed be due to rusting stucco wire or nails embedded within the various layers of stucco. If your stucco has remained in good condition for 40 years and the stains are newly appearing, it is unlikely that outside moisture is only now getting behind the stucco and causing damage. This may be the case if eavestroughs or roofing materials are damaged and leaking on the walls or another outside component of the house is deteriorated. The exterior of the house should be inspected to ensure that this is not the case.
The black stains on the wall are more likely caused by moisture escaping into the wall cavity from the interior. This may be due to a change in the home such as upgrades to windows or insulation or heating system. It will be much more difficult to determine the exact cause in this situation, but look to newly installed components or systems that may have raised the moisture level in the home or closed up old gaps that easily allowed air to escape to the exterior. If you have done recent upgrades, added ventilation to the interior, to reduce the relative humidity in the house, may prevent further staining.
To answer your question more directly, you may choose to paint the exterior stucco, no matter what the cause of the stains, and this should be approached with several things in mind. Stucco is a cement-based material that is quite “breathable”. This means that moisture that does get behind the stucco due to rain or from ex-filtration through the walls will readily escape and prevent moisture damage to the wall. Painting the stucco with the wrong material may reduce this property and cause more moisture problems and further stains. The newly installed paint may also bubble and peel as the moisture tries to escape the building envelope, as it did in the past.
The most important part of any paint job is the preparation before any paint is applied. With stucco, there may be years of dirt on the surface or embedded in the material. This must be well cleaned if any paint is to properly adhere to the stucco. A pressure washer is normally used to thoroughly clean the stucco, which then must be allowed to fully dry before proceeding. Small hairline cracks or damaged areas may then be patched with cement-based patching compound or caulked with masonry caulking. If larger areas are damaged, metal lath or stucco wire will have to be installed to ensure proper adhesion of the new patching material. Several layers of patching may be required, similar to original installation of the stucco. Once the surface is prepared to this stage, you can proceed with the finish.
As you suggested, the stains should be sealed to prevent bleeding through the finish coat of paint. There are now many different types and brands of primer-sealers on the market, that may be appropriate for use on stucco. Inquiring with the paint experts at your paint supply store or home centre will provide the answer for the best product for your home. You may be able to find a primer that you can apply to the entire exterior of the stucco that will seal the stains, as well as providing a proper surface for the finish coats of paint. Any unpainted surface requires a good quality primer to ensure the new paint will not peel. Once this is fully applied good quality, breathable stucco paint may be rolled, brushed, or sprayed on the entire home. Caution should be taken to avoid “bargain” stucco paints as they may have a limited life expectancy and may require recoating in only a few years after initial application.