Clean Stucco

Exterior stucco is a porous material with a textured surface. Stucco’s porosity causes it to stain quickly and the textured surface allows dirt, grime and debris to settle easily into the crevices. The most common stains on exterior stucco are dirt, mildew, mold and algae. These stains can be effectively removed by using the proper tools and procedures. Because stucco is somewhat fragile, care must be taken during the cleaning to prevent damage from occurring.

Steps

Inspecting the Stucco

1. Examine the stucco for chips or cracks. Before you clean stucco, you must examine it closely for any flaws. If you wash damaged stucco, water can get into the cracks, causing mold and mildew to grow, among other issues. If you locate chips and cracks in your stucco, don’t move forward with cleaning it until you’ve repaired the damage.
   • You can repair minor damage (small chips, hairline cracks, etc.) yourself.
   • Damage beyond that is best repaired by a professional who is experienced with stucco work.
2. Seal the cracks and chips with caulk. If you find relatively minor flaws in your stucco, seal them yourself using a color-matched exterior acrylic caulk. You can get this caulk at most home improvement stores. It might be difficult to find a caulk that's the exact same color as your stucco, but buy the closest you can get. As long as the repairs are minor, a slightly off-color caulk probably won’t stand out.
   • To replicate the texture of the surrounding stucco, try pressing sand or another gritty substance into the surface of the wet caulk.
   • Give the caulk at least a week to dry completely before you attempt to clean the wall.
3. Evaluate the problems areas. What’s causing the stains and discoloration on your stucco? Is it dirt and general debris? Or it something else, like mildew or algae? Mildew appears as a black stain that looks a lot like dirt. If the black stain occurs near a place on the wall where water often drips or accumulates, such as under eaves, the black stains you see are most likely mildew, not dirt. If you have green staining, this is algae.
   • You will need to use bleach to clean off micro-organisms like algae and mildew.
   • If you’re dealing with significant staining over large areas of wall, consider having a professional clean the stucco.

Applying the Cleaning Solution

1. Mix a cleaning solution of concentrated cleaner and bleach. Buy a concentrated cleaner made specifically for use on exterior surfaces. These concentrated cleaners are available at any home improvement store. Look for products that say "professional strength concentrate" and include ingredients like acetic acid and sodium o-phenylphenate. Read the instructions carefully. They will tell you how to mix up the cleaning solution for that particular product. Most concentrated exterior cleaners are activated by adding household bleach and hot water.
   • The bleach activates the solution and also helps to kill micro-organisms like mildew and algae.
   • Wear eye protection and rubber gloves when preparing and using the cleaning solution.
2. Mix a cleaning solution of Borax and dish soap. If you want to avoid bleach, try Borax-based solution instead. Borax is a natural substance that is less toxic than bleach and still effective at killing mold. For this cleaning solution, you’ll need three ingredients – hot water, Borax and dish soap. Mix the following amounts in a large bucket:
   • 2 gallons (7.57 L) of warm to hot water
   • 2 tablespoons (29.5 mL) of dish soap
   • 1/2 cup (118.2 mL) of Borax
3. Use a pressure washer to saturate the wall with water. Pressure washers come with different nozzles that spray with varying amounts of water pressure. Stucco is delicate, so attach a low pressure spray tip nozzle to the pressure washer’s hose. Saturate the entire wall with water before you begin applying the cleaning solution. To prevent damage, make sure the water hits the wall at a 45 degree angle.
   • The nozzle should be held at least 24 inches away from the wall. Apply the water in an even stream.
   • If you don’t own a pressure washer, you can rent one from most home improvement stores for about $75 per day.
4. Spray on the cleaning solution. Fill a single pump sprayer with the cleaning solution. Divide your wall up into workable sections. Use the pump sprayer to squirt the cleaning solution on the first section of the wall. Start at the bottom and work your way up to the top until you’ve saturated the entire first section of the wall.
   • Focus on areas with heavy stains.
   • Pump sprayers can be purchased at home and garden stores. They come in various sizes – get one with a one to two gallon tank.
5. Allow the cleaning solution to sit for five to ten minutes. This will ensure the cleaning solution penetrates the stains deeply. Spray the section lightly with water once or twice while it soaks in the cleaning solution. Stucco is very porous and will soak up water, so after a few minutes you’ll need to add a little more water to keep the detergent activated.
   • If you are dealing with very stubborn mold, algae or mildew growths, you may need to allow the solution to sit for up to 30 minutes.

Scrubbing and Rinsing the Wall

1. Use a rotating scrub brush attachment on the heavily stained areas. Areas with rust and other heavy stains may need a little extra scrubbing. While the wall is still saturated in the cleaning solution, use a high quality rotating scrub brush attachment to work on the heavy stains.
   • This will loosen up the grime and debris and make it easier to rinse off.
   • If you’re dealing with rust, scrub it slowly and carefully.
2. Rinse the cleaning solution off the first section. Attach the low pressure spray tip nozzle onto the pressure washer’s hose. Rinse the solution off the first section of the wall, starting at the top and working your way down to the bottom. You will be able to see the dirt moving down the wall once you begin at the top. Follow that line of dirt down with the hose, making sure you guide all of it to the bottom.
   • Rinse the wall thoroughly so that no cleaning solution remains behind.
   • Finish one section completely before beginning the next one.
3. Saturate the next section of the wall with cleaning solution. Repeat the same actions as before and allow the solution to sit for five to ten minutes before you rinse it off with the pressure washer. Continue in this manner, section by section, until you’ve completed the entire wall.
   • Always rinse off the cleaning solution, working from the top to the bottom, to properly get rid of all the residue.
4. Do a spot check. Examine the stucco carefully. Check to make sure you haven't missed any areas of dirt. If necessary, repeat the application of cleaner and pressure rinsing to remove additional stains. Allow the wall to dry for at least 24 hours.

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