How to soft wash sand and cement render

October 04, 2019

How to soft wash sand and cement render


Sand and cement render can offer a building excellent protection from the elements.

This type of render does not have to be painted and skilled soft washing treatments will make it look considerably better.

When soft washed, older properties – like the one below – in particular can look very appealing with a plain sand and cement finish.

And Benz "Clean & Maintain" soft washing systems – will keep the property permanently clean.

Softwashing sand and cement render

The biofilm of algae and lichens that grow in sand and cement render, keeps the render damp and hence has the potential to damage the structure of the building.

Removing the biofilm results in a structural sound and aesthetically pleasing unpainted finish.

Sand and cement render must be correctly maintained and cleaned for three important reasons:

1. In preparation for painting unpainted sand and cement render

2. Re-painting previously painted sand and cement render

3. Maintenance of unpainted sand and cement render


1) Softwashing sand and cement render ready for painting

In our experience soft washing sand and cement render in preparation for painting presents some challenges.

In particular, the heavy black algae and lichens that grow into sand and cement render can be deeply rooted and therefore very difficult to remove.

And because it is not advisable to paint over these growths – because they will regrow under the layer of paint – they must be killed and removed prior to painting.

Why do professional painters need sand and cement render to be cleaned quickly?

One of the main reasons is because professional painters only have a "weather window" of spring, summer and early autumn to complete most of their work.

Therefore cleaning contractors must must plan their work in advance, in collaboration with the painter, when giving a soft washing treatment of sand & cement render that is going to be painted.

This is an easy matter if the render is only a few years old. Just treat it with Lightning Cleanze at 5:1. Simply spray, brush the product into the worst areas of colonisation, and then thoroughly rinse.

The sand and cement render can be painted once it has fully dried out.

How to soft wash sand and cement render that is over five years old

Old sand and cement render will have a heavy biofilm that will be difficult to remove using Lightning Cleanze alone and during just one visit to the property. 

Experience has taught us to treat the render with Benz Bio Cleanze first. Then allow at least two months before giving it a final treatment with Lightning Cleanze.

This has proved, after several years testing various techniques of soft washing sand and cement render, to be the easiest and most effective way to prepare the render for painting.

This is especially true when the customer wants the fastest possible results.

Quick tip: If the render has not been treated for a very long time, say ten years or more, we've found the best practice is to allow Bio Cleanze (applied at 20-50:1 dilution depending on the severity of the growths) four months to work before treating with Lightning Cleanze.


Here's an example of an extremely contaminated 35 year old render before soft washing:

Heavy biofilm on sand and cement render

Here's the same render four months after a soft wash treatment using Bio Cleanze:


And the same render after a final soft wash treatment using Lightning Cleanze:

Sand and cement render after softwash treatment with Benz Lightning Cleanze

Quick tip: In extreme cases like this the white lichen dots remaining on the surface may take a further 4 – 8 months to weather from the surface. 


The importance of co-operative planning

The various technical, environmental and diarying factors involved, requires co-ordination between the cleaning contractor, painter and customer.

This requires some skilful organisation but the final result will be superior to – and far easier and more effective than – multiple applications of Lightning Cleanze.

Quick tip: Ideally apply Bio Cleanze in the winter so the sand and cement render is ready for final cleaning and painting in the spring/summer


2) Softwash cleaning of already painted sand and cement render in preparation for re-painting

When sand and cement is already painted, the most effective method will usually be to treat with Lightning Cleanze, often at only 10:1 dilution.

In the photo below you can see an example of a painted sand and cement render before being given a soft wash treatment:

Painted sand and cement render before softwashing

A small job like this can frequently be completed in one day, simply by treating with Lightning Cleanze, as you can see in the photo below:

Treating painted sand and cement render with Benz Lightning Cleanze

... allow the softwash chemical to activate, then thoroughly rinse it off. You can see the immediate result in the photo below:

Sand and cement painted render after softwash cleaning with Benz Lightning Cleanze

Click here to read Trade Tips articles about soft washing painted surfaces

How to rinse already painted sand and cement render:

We suggest rinsing with a power washer set to a very light pressure. Regular hose-pipe pressure at the point where the jet hits the surface is ideal. This will remove any chemical residues, spiders nests, birds droppings, dust, etc, all of which is essential for good pre-painting preparation.

If there's flakey paint on the render, rinsing will usually remove it. But you don't want to take off good paint. Hence the reason we suggest low pressure rinsing.

Obviously, what will come off, will come off and your customer needs to expect that. So it's important to educate your customer in advance as to what may happen, so they know what to expect.

Quick tip: As you can see in the photo below, soft washed sand and cement render that has been painted will normally further brighten through time. This is because any remaining dead growths will be removed from the surface by the  natural weathering effects if rain, wind and sun.

Softwashing painted sand and cement render three weeks after cleaning with Benz Lightning Cleanze 

3) Softwash cleaning and maintenance of unpainted sand and cement render

If the render is not going to be painted then it can be left to self-cleanse through time by treating with Bio Cleanze. The biological growths will usually disappear over a period of months.

Sometimes, when the sand and cement render has not been cleaned for many years, the full impact of soft washing with Bio Cleanze may take up to a year to show full results.

Quick tips:

  • If your customer wants the fastest results, and a sand and cement render has not been previously treated with a Benz biocide, allow at least two months after treating it with Bio Cleanze before giving it a final clean with Lightning Cleanze.
  • If the render has not been treated for a very long time, say ten years or more, we've found the best practice is to allow Bio Cleanze (applied at 20-50:1 dilution depending on the severity of the growths) four months to work before treating with Lightning Cleanze.
  • In general treat with Bio Cleanze first only if the sand and cement render is unpainted. This is because Bio Cleanze will get deeper into the rough porous surface and will kill spores deeper into the substrate than can Lightning Cleanze or Tornado Cleanze (or any sodium hypochlorite based product or mix).
  • Another treatment of Bio Cleanze after 6-12 months will restore the full residual protection against re-colonisation that is such a big plus factor of Bio Cleanze (because the post-treatment with Lightning Cleanze, combined with the heavy work it had to do when first killing the heavy growths, reduces the residual effectiveness of the initial Bio Cleanze treatment).

What's different about soft washing sand and cement render compared to soft washing pre-coloured renders such as KRend?

We're often asked to clean sand and cement render in preparation for painting, but are rarely asked to do this for KRend and other pre-coloured renders.

This is because, for easy and cost-effective maintenance, most home owners choose to stay with the original colour of their render.

Quick tips:

  • If, when cleaning KRend and other pre-coloured renders, there are still a few marks left after the initial treatment, no worries as they will usually disappear over the coming months. The worst case scenario is that a second treatment with Lightning Cleanze may be indicated.
  • However, these marks are much harder to get rid of on sand and cement render. And it's not a good idea to attempt painting over these marks. * See below for our suggestions for how to get rid of deeply ingrained staining/marks.
  • If a pre-coloured render has been left for several years without being treated, and the roots of the algae, lichen, etc have grown in deep, treating using the same approach as for sand and cement render may be indicated.
  • By adopting the Benz "Clean & Maintain" approach to ongoing maintenance you will be able to offer your customers a property that will be permanently clean, as well as securing long-term repeat income for yourself.

How does the difference between sand and cement render and pre-coloured render change our approach to soft washing render?

The main difference between soft washing pre-coloured render and and sand and cement render is that we treat it with Bio Cleanze first.

We leave the treated surface as long as is practical (preferably several months), then give it a final treatment of Lightning Cleanze.

The reason is that Bio Cleanze has the ability, because of its chemical formulation, to soak in more deeply than Lightning Cleanze (or any SH mix). When biological organisms have been allowed to grow into the surface for many years their roots and spores can be deep in the substrate, requiring the deep penetration of Bio Cleanze.

This sequence of soft wash treatment will:

a) Leave a sand and cement render perfectly clean.

b) Leave the property ready for painting once the sand and cement render has fully dried out. This could be from a few days to a couple of weeks depending upon the prevailing weather.

Quick tips:

  • The above method is similar to how we soft wash other very heavily stained renders
  • The principles described above are the same as for soft washing pebble dash (dry dash) render

Click here to read the Trade Tips post on how to remove heavy red algae staining from wall render

Quick tips:

  • The difference from the above Trade Tips post is that, when soft washing sand and cement render we leave the softwash biocide on the substrate for as long as is practical.
  • Following treatment with Bio Cleanze we usually allow for two brushed-in applications of Lightning Cleanze at 5:1 to get any remaining black marks out.

    This does not use much product but is labour intensive because of the need for multiple rinsing. Therefore Tornado Cleanze can be the more effective choice for the worst affected areas. This is because it's a stronger chemical that produces faster results on badly colonised surfaces.


    Offer your customer an ongoing maintenance programme

    Put your customers on a "Clean & Maintain" ongoing maintenance programme consisting of regular soft wash treatments of Bio Cleanze every 2-3 years or at the first signs of re-colonisation (exact timing depends on the local environment). 

    Putting customers on an ongoing maintenance programme is a win-win for both you and your customers.

    An ongoing maintenance programme gives your customers a permanently clean property, which they love, and gives you a reliable source of repeat business.


    Tell your customer what to expect – in advance of doing the work

    We've found that when a contractor clearly explains to their customer the nature of the work they are going to undertake for them – so their customer understands the problems a particular job presents, how the contractor intends to address those problems, and what results can be reasonably expected – customers rarely have any concerns.


    We hope you've found this information helpful and sincerely wish you great success in your soft washing business,

    Ben, Leo, Fiona, Stevie & Anthony


    Click here to explore the full range of Benz softwash cleaning products

    Benz Softwash Bio Cleanze DDAC biocide & Benz Softwash Lightning Cleanze SH sodium hypochlorite biocide


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