Cleaning brick can be as simple as hosing it down, or it can be quite a production. You will need to have a water hose and nozzle, safety goggles, rubber gloves, household bleach and a plastic bucket to clean some stains. Others will require a Shurclean or muriatic acid solution, an acid brush with a long handle, a plastic mat, old box or carpet square, and particularly stubborn staining may require a pressure washer. Be sure to read the directions of all cleaning agents carefully to ensure the safety of your brick surfaces as well as your safety.
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Brick can be dirty or discoloured for different reasons. Cleaning brick that is stained by algae, mildew or mould will require different cleaning solutions than rust stains or mortar and cement marks. Algae, mildew and moulds leave a greenish, black or white spotted appearance that sometimes has a raised finish. Rust is the same orange-coloured mark as it is on metal finishes. Mortar or cement marks will be the same colour as the grout or as cement found in a sidewalk. When cleaning brick with anything, use gloves and safety goggles for your protection. Don't clean bricks with anything but clear water in direct sunlight or when there is any wind that could blow toxic solutions around. Always work from the top of the wall down to prevent clean areas at the bottom from getting dirtied as you clean the top. When cleaning a large area, work on one section at a time. Be sure when cleaning with more than water that the brick remains good and wet the whole time.
Clean off mould and mildew with bleach. Mix up a bleach-and-water solution. Do this outside or with all the doors and windows open as this can create fumes. If the stains are small, use gloves and a small scrub brush. Use a circular motion and scrub with lots of the solution. Let the solution sit for a while and do some of the work for you. For a large section, pour the bleach and water into a pump sprayer like the kind used in gardening. Soak down the surface with plain water from the hose. Get it good and wet. Starting at the top of the wall, spray the bleach and water solution--again, soaking it completely. Leave the bleach water on the wall for several minutes but don't let it get dried out. After five minutes or so, use the hose with a sprayer nozzle on it on a small test area to see if the solution has cleaned it. Spray near the bottom so the clean water doesn't dilute the rest of the brick if it hasn't been on there long enough. If this method yields no results, try using straight bleach and an acid brush with a long handle which you can buy at home improvement stores. Keep the brick good and wet. Don't allow the bleach solution or straight bleach to dry on the brick or it could cause discolouration.
For rust or mortar stains, you'll have to use a product that's acid based such as Shurclean which you can buy at home improvement and hardware stores. You can also use muriatic acid, but the solution is easier. Use a mixture of one-fourth acid or Shurclean to three fourths water in a plastic bucket. Make sure that the bucket is only filled about halfway, as this solution will discolour anything it touches. To be on the safe side, put the bucket on a rubber mat or inside a box to catch any small drips. Soak the brick down well with a hose. Apply the acid solution with an acid brush and be sure to use gloves to protect your hands. Scrub well and let the acid sit for about 15 minutes. Test a small patch at the bottom of the brick by rinsing it well with your hose and spray nozzle. If it's clean, rinse the entire wall really well. you will need to stand there and rinse the wall for a good while--maybe 15 to 20 minutes. It's important that you get all of the acid off or it will eat at your bricks and mortar.
If the above solutions don't work or they aren't something that you are able to do, try renting a power washer. The force of the water sometimes does a better--and safer--job than even the acid solutions can. Just make sure to ask questions about the sprayer you'll be using. They aren't all the same. Generally it's best to keep the pressure between 1000 and 1200 psi. Use a fan nozzle rather than a pinpoint spray for easier coverage and less chance of damage. Test a small area to see if damage occurs. Remember that the older the brick is, the more likely it is to be damaged by water pressure. Find out how close to the brick you should stand, how the handle operates and any precautions or safety features you should know about. Don't ever let children play with these pressure washers as they can cause serious injuries to people if they're sprayed with them. Be careful also not to leave the pressure washer on one spot for too long as it can leave etch marks. Keep it moving the whole time you're using it.
If you get the brick clean you should probably seal it to prevent ever having to do this again. Get a silicone sealant or a siloxane sealant at any home improvement or hardware store. Follow the directions carefully and you will have a beautiful protective coating on your brick that will prevent it from becoming stained again.