How to clean exterior render

Inspired by a Softwashing UK guide. This article draws on the video “Softwashing K Rend: The Ultimate Guide” from Softwashing UK (softwashinguk.co.uk), a UK specialist in exterior cleaning for render, stone, and timber. Their explanation of why high-pressure washing damages modern thin-coat render is particularly useful - it is a common mistake that causes cracking and delamination.

1. Identify the render type before you start

This matters more than most people realise. Traditional sand-and-cement render is fairly robust and can handle moderate pressure. Modern thin-coat polymer renders - K Rend, Parex, Silicone K1 and similar - are much more delicate. High-pressure washing will blast the aggregate out of the surface, leaving it rough, porous, and far more likely to pick up algae again quickly.

If the render was applied in the last 20 years and has a slightly textured or grainy finish, treat it as thin-coat and go low-pressure or soft wash. If you are not sure, start low and work up. You cannot un-damage render you have already blown apart.

2. Wet the wall and protect plants and fittings

Before applying any cleaning product, wet the wall down with clean water. This pre-wetting stops the biocide being absorbed too quickly into a dry surface. It also helps the product spread evenly.

Cover nearby plants, cover any soil beds at the base of the wall, and put masking tape over outdoor light fittings, meters, and taps. Some biocide products are harmful to plants at full strength - check the data sheet for your chosen product.

3. Apply a biocide or render cleaner

A biocide kills the algae, lichen, and mould rather than just washing it off. This is what stops it coming back as quickly. Dilute the biocide to the manufacturer’s recommended ratio (usually 1:10 to 1:20 with water) and apply it with a low-pressure garden sprayer, working from the bottom of the wall up to prevent streaking.

For heavy green growth, you may want to apply it undiluted at the worst patches. Products such as Algon, Wet & Forget, or specialist render cleaners from companies like Kintsugi all work well on UK render. Check the product is safe for your render type before applying.

4. Allow dwell time

This is the step people are most tempted to skip, and the one that makes the most difference. Leave the biocide to dwell on the surface for at least 30 minutes - an hour or more for heavy algae. The product needs time to penetrate and kill the biology at the root.

Some products work better if left overnight and rinsed the following day. Others produce a visible result within the dwell time as the algae turns grey-brown and dies off. Either way, do not rinse too soon.

5. Rinse at low pressure

Once the dwell time is complete, rinse the wall with clean water. On traditional sand-and-cement render, a garden hose with a jet nozzle or a pressure washer on a low fan setting (below 80 bar) is fine. On thin-coat or polymer render, use a garden hose only - no pressure washer.

Work from top to bottom this time to flush the dead material downward and off the wall. The algae and mould should rinse away fairly easily if the biocide has worked properly. If patches remain, re-treat rather than switching to higher pressure.

6. Deal with stubborn stains

Some staining - particularly orange-brown iron staining, efflorescence (white salt deposits), or deeply embedded lichen on traditional render - will not respond to a standard biocide. Iron stains can be treated with a specialist oxalic acid-based product. Efflorescence usually just needs a stiff dry brush once it has dried out; do not wet it or you will dissolve it back into the surface.

Lichen is harder to remove than algae and may require a second or third application of biocide over several weeks. It is alive right down to the hyphae, so surface rinsing alone will not shift it.

7. Consider a protective treatment

Once the render is clean and fully dry (allow a week or two in reasonable weather), a render sealant or silicone water repellent treatment applied to the surface will slow the return of algae considerably. Water-repellent treatments do not make the render completely resistant to biology, but they reduce the moisture the algae needs to establish. A single application typically lasts three to five years.

It is worth doing, particularly on north-facing or shaded walls that stay damp longest.

When to call a handyman

If the render is cracked, blown, or coming away from the wall, cleaning it will not help and may make things worse. Richard can assess whether the render needs patching or replacement before the cleaning work is done, and can arrange soft-wash cleaning for larger jobs in Sandwich and across East Kent.