Before painting or treating the outside of your house, you need to make sure the stone or brickwork is clean and free from lichen, mould and Efflorescence.
New brickwork or stonework should be left for at least three months before any treatment is applied. During this time a white powdery substance may seep out of the bricks, stone or cement. This is caused by the water dissolving the salts found within all of these materials. If Efflorescence appears on older brickwork, it is often a sign of excess moisture, and the source of the damp needs to be found before it is treated.
To remove efflorescence, wait for a dry day and brush with a stiff-bristled brush. Continue to do this at intervals until the deposits stop appearing. Do not try to wash the wall as this will just make the salts soak back into the brick or stone and reappear later. Once no more deposits are forming, the wall is dry and can be treated with weatherproofing solution if required. If you plan to paint the brick or stonework, seal the surface with a Alkali-resistant primer first.
There are several types of mould and lichens which will form on brickwork and stonework, particularly in damp conditions. These appear as tiny specks of varying colours (green, yellow, white and black to name a few) which gradually spread to form larger patches. Before you remove these stains, you need to figure out what is causing the damp which has allowed them to grow.
Walls which get little or no sun may suffer from organic growth. This can sometimes be relieved by cutting back overhanging trees and bushes to increase natural ventilation. If the mould or lichen is concentrated in a vertical strip down the wall, a leaking gutter might be the problem. Finally, check that your damp-proof course is not being bridged by earth or debris piled against the wall.
Once you have found and fixed the cause, you can start to remove the mould and lichen patches. Wait for a dry day and brush the patches with a stiff-bristled brush. This can be a messy, dusty task, so wear a face mask and goggles to protect yourself. When the surface patches are gone you need to treat the wall with a bleach or fungicidal solution to kill the tiny spores buried in the brickwork.
Using normal household bleach, mix one part with fours parts water. Use and old paintbrush to completely cover the effected area. Leave this for a couple of days and then scrub the area with clean water. In very bad cases of fungal growth, it is worth repeating the whole bleaching process once more.
This can be purchased from most DIY stores for a few pounds. Follow the dilution instructions (these vary depending on brand) and then apply to the effected area with an old paintbrush. Leave for 24 hours and scrub the wall with a stiff brush and clean water. For very severe cases of mould, you may need to repeat the process one or two more times to finally fix this problem.