Coloured Render: Can It Be Cleaned Without Patching the Finish?

This is the question we get asked most often about coloured render, and it is usually asked nervously: the wall has gone green or grey in places, the owner wants it dealt with, and they are worried that cleaning it will leave them with something worse than they started with. It is a fair worry. We have seen plenty of walls where exactly that happened.

What “through-coloured” actually means

Most modern render on Irish houses is through-coloured. The pigment is mixed into the material rather than painted on afterwards, so the colour runs through the full thickness of the coat instead of sitting on top of it as a skin.

That matters for your question, because it means there is no layer of colour to lift. You are not in the position of someone with a painted wall, where the finish is a coating that can be stripped. The colour and the render are the same thing.

The real risk is abrasion, not colour loss

If colour is not the thing at risk, what is? The surface itself. Render has a texture that was applied deliberately, and a face that has cured. Take a pressure washer to it and you cut into that face: you open the surface up, you change how it holds light, and you remove material unevenly depending on where the lance was held and for how long.

The colour has not gone anywhere. The wall simply reflects light differently where it has been cut into, and your eye reads that as a lighter or darker patch. On a through-coloured render the patch is not a bare spot, it is a roughened one, which is why it does not “wash back in” afterwards.

There is a second cost that shows up later. A surface that has been opened up holds moisture more readily than one that has not, and moisture is the thing algae needs. That is the mechanism behind the pattern we cover in why render goes green: the wall comes back greener, sooner, than it would have.

Why patchiness usually starts with a pressure washer

Almost every patchy render we are called to look at has the same history. Somebody cleaned the worst bit. Perhaps a contractor did one elevation, perhaps the owner borrowed a machine and did the section they could reach from the ground.

The result is a wall with two different surface conditions on it. The treated area is not cleaner in the way people expect it to be, it is textured differently, and the boundary between the two is visible from the road. On a uniform colour that boundary is unmissable, because there is no variation in the finish to hide it.

This is also why the problem is worse on newer houses. A modern silicone or thin-coat render is deliberately consistent, so anything that disturbs that consistency stands out immediately. An older, more variable cement render is more forgiving of a bad patch simply because it was never uniform to begin with.

Treating the whole elevation, not the visible patches

The approach that keeps a coloured render even is to treat the entire elevation in one go, rather than attacking the spots you can see.

There are two reasons. The obvious one is that a whole wall treated the same way stays the same colour across its width. The less obvious one is that the clean-looking parts of a green wall are rarely clean. They are behind the visible areas on the same curve, and if you deal only with what shows, the rest catches up within a season and you are looking at a patchy wall again for a different reason.

Because the work is chemical rather than mechanical, nothing is being cut away. The growth is killed and then weathers off the wall over the following weeks, which is slower to look finished than a jet wash but leaves the render face as it was. The difference between the two approaches, surface by surface, is set out in soft washing vs pressure washing.

When we test a section first

On anything we have not worked with before, or where there is a reason for doubt, we treat a discreet section first and come back to it before committing to the whole wall. A gable return, a section behind a downpipe, somewhere that is not the first thing you see from the gate.

Reasons for doubt include a render we cannot identify, a wall that has clearly been cleaned before by someone else, a finish that is already showing wear at the base, or a coating that turns out to be paint over render rather than render itself. That last one is more common than people expect, and it changes the job.

What we cannot fix

We would rather say this plainly than have you find out afterwards. If a wall has already been abraded, cleaning the rest of it will even out the biological growth but it will not even out the surface. The patch will be less obvious once the whole elevation is the same colour of clean, and in some cases that is enough to satisfy people. In others the texture difference still catches the light and the honest answer is that the finish has been altered.

Render can be re-coated, and that is a rendering job rather than a cleaning one. If that is where a wall has ended up, we will say so instead of taking money for a clean that cannot deliver what you are hoping for.

Common questions

Related services

• Softwashing for render and walls
• Roof cleaning
• Gutter cleaning