The three major factors affecting the appearance of external brick walls are: white powdery stains known as “efflorescence” caused by salts or lime leaching out of damp brickwork, darkened or stained brickwork caused by airborne pollution, and graffiti. Each condition requires a different cleaning method. While efflorescence can be brushed off, pollution-born stains and graffiti are harder to eliminate. These stains or marks are best removed by either treating the surface with acid or using a graffiti-removing gel.
Scrub efflorescence off with a dry, stiff-bristled scrubbing brush; this will break the crystallized deposits into a powder. Do not hose the wall down. Wetting the wall will simply allow the efflorescence to migrate back into the surface.
Brush the powdery deposits from the wall with a soft paintbrush or broom. If the white deposits are obvious streaks emanating from damaged flashing, leaky down-pipes or from water overflow pipes, repair where necessary or adjust the water level in your water heater tank. This will prevent efflorescent deposits from re-forming.
Prepare the wall for acid treatment or pressure washing. Close all windows. Tape plastic bags or plastic sheeting over electrical outlets. Place a chemically absorbent mat in the catch basin of the runoff drain servicing your property to protect the environment.
Make up a solution of 20 to 40 grams of oxalic acid per litre of water to remove green or yellow vanadium stains. Scrub briskly with a stiff bristle scrubbing brush to spot clean. Sluice down thoroughly with a garden hose.
Remove dark pollution-born stains from a brick wall. Scrub the entire wall with a solution of one part of hydrochloric acid to 20 parts of water when cleaning light coloured bricks and one part of hydrochloric acid to 10 parts of water when cleaning dark coloured bricks. Prepare the mixture in a plastic bucket and scrub briskly with a stiff bristle scrubbing brush.
Start at the top left-hand corner and work across the wall. Progress downward in overlapping layers. Treat one square meter at a time and hose each area down with an electric-powered pressure washer before moving on to the adjacent section.
Pressure-clean the wall with and electric-powered pressure washer fitted with a wide spray nozzle. Set the outlet pressure to between 900 and 1,000 p.s.i. with a flow rate of 8 to 10 litres per minute. Do not be tempted to adjust to a higher pressure as this will damage brick surfaces and bonding cement. Stand at least 3 feet from the wall during operation. Hold the nozzle at the distance from the wall specified in the operating manual. Keep the nozzle moving and use a wide sweeping action.
Go over the entire wall with the pressure washer after the initial cleaning operation. Sluice off all traces of hydrochloric acid; any residue left on the wall will lead to vanadium stains.
Smear a thick layer of citrus-based graffiti-cleaning gel over all spray-painted graffiti and work it into the surface with a stiff nailbrush. Wipe off with a rag and add more gel. Cover with plastic sheeting taped to the wall. Leave overnight to allow the gel to work its way into the surface.
Pressure clean with a wide spray nozzle with a flow rate of 8 litres per minute and a pressure of between 900 and 1,000 p.s.i. Clean graffiti off as soon as it appears. This will make it easier to remove and also discourage further vandalism. Older graffiti may require two or three cleaning operations.
Read the pressure washing manual carefully before use.
Wear goggles, rubber gloves, old clothing and a respirator while working with acid. Never more than 1 part of hydrochloric acid per 10 parts of water; a stronger solution is dangerous to work with, and it will damage certain types of brick. Read the pressure washing manual carefully before use.