Black staining on rendered walls

Black staining on rendered walls

There are many more contaminants which can cause damage, discolouration and stains to rendered surfaces. Treatment may differ for each of these types, the trick is to recognise the problem. Treatment with a biocide wash is an important “catch-all” but sometimes, the need to scrub, brush and agitate micro-organisms is crucial.

Visible bacterial staining is rarely one microorganism. Fungi and algae have a symbiotic relationship. The black is usually a filamentous fungus, commonly seen under stone joints, windowsills or coping stones, if left untreated it does spread over flat surfaces.

To begin with, sunlight (UV) is the enemy of bacteria – so hanging your whites out to dry on a sunny day leaves dry clothes smelling fresh and clean, sunshine is natures disinfectant. It is also the reason most bacterial growths occur on the north-facing side of structures, where there is less sun.

As fungi spread the mycelium, (the vegetative part of a fungus colony), ages and, begins to act like sunscreen protection from UV as it metabolizes into this black crusty paste, allowing algae and fungi growing underneath to thrive, fortunately, it is water-soluble and can be dissolved.

A biocidal treatment with GK Pro will work to remove the black staining but once allowed to establish this process can take many months, the black will continue to lessen and become a mere shadow of its former self. This is a natural process, unfortunately, it does take time.

Once the surface bacteria have been treated with biocide it is no longer viable (its dead). Any further treatment can only be to remove any staining which remains, I suggest one of two options:

Steam cleaning is a good way to clean dirty and discoloured render. (This type of treatment is often used for historic buildings as it is non-invasive) Many systems exist, be sure to use something equivalent to THE THERMATECH ‘SUPERHEATED WATER’ SYSTEM the key benefit is that ThermoTech is adjustable from 20 to 140 bar water pressure and can be used with pressure-reducing guns to attain pressures lower than 20 bar plus the temperature is fully adjustable up to 150°c. This combined with a nozzle fan pattern of between 40° to 60° makes it ideal for gentle cleaning of the most delicate substrates including through colour and thin coat monocouche renders or EWI systems.

Alternatively, removal with an Alkali gel to break down the black crusty coating, (we have one GK Black Spot Remover), usually applied via an extending pole with a soft brush head, agitated and left to dwell for up to 24 hours, then rinsed with water – Your choice is the harder physical work and longer labour costs versus time saved in kit hire. The scale of need usually determines your answer.

Written by Mike Witherspoon of Rabart Decorating Centres: In South Wales and South West England Rabart ASM’s are able to visit a site and tailor building-specific performance specifications allowing you to tender for works precisely.