How to softwash sand and cement render

October 04, 2019

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

This type of render does not have to be painted but skilled softwash treatments will make it look considerably better.

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

And Benz "Clean & Maintain" softwash 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, the softwash cleaning of sand and cement render in preparation for painting presents some challenges.

In particular, the heavy black algae and lichens that grow in 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 removed prior to painting.

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

One of the main reasons professional painters need sand and cement render cleaned quickly is because they 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 of giving a softwash treatment.

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 by biological growths, and then thoroughly rinse.

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

How to softwash clean sand and cement render that is over five years old

A sand and cement render that is over 5 years old will have a heavy biofilm that will be difficult to remove using Lightning Cleanze alone during one visit. 

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

This has proved, after several years testing various techniques of softwashing sand and cement render, to be the easiest and best 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 softwash cleaning:

Heavy biofilm on sand and cement render

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


And the same render after a softwash treatment using Lightning Cleanze:

Sand and cement render after softwash treatment with Benz Lightning Cleanze

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

The only alternative to this would be to use a pressure washer with the problems of access that entails, and at the risk of damaging the render.

The need for co-operative planning

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

This requires a little 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 softwash 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 thorougly 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 softwashing painted surfaces

How to rinse already painted sand and cement render:

We suggest rinsing with a power washer set to a very light pressure. 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, softwashed 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 natural weathering.

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 or Perma 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 softwashing with Bio Cleanze or Perma 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 or Perma 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 / Perma 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 softwashing sand and cement render compared to softwashing 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 is clean in perpetuity, 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 softwashing render?

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

We leave the treated surface as long as is practical (preferably several monthis), then give it a final treatment of Lightning Cleanze or Tornado 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 can go deep, requiring the deep cleaning action of Bio Cleanze.

This sequence of softwash 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 softwash very heavily stained render.
  • 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 softwash cleaning sand and cement render we leave the softwash biocide on the surface for as long as is practical.
  • Following treatment with Bio Cleanze or Perma 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. So Tornado Cleanze can be the more effective choice for the worst affected areas because of it is a stronger chemical that produces faster results on badly colonised surfaces.


    Offer your customer an ongoing maintenance programme

    Put your customers onto a "Clean & Maintain" ongoing maintenance programme consisting of regular softwash treatments of Bio Cleanze or Perma Cleanze every 2-3 years (exact timing depends on the local environment). 

    Putting customers onto an ongoing maintenance programme is a win-win for both customers and softwash contractors.

    An ongoing maintenance programme gives customers a permanently clean property, which they love, and the softwash contractor a reliable source of repeat business – which they also are also very happy about.


    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 the results that can be reasonably expected – customers rarely have any concerns.


    We hope you've found this information about soft washing sand and cement render helpful,


    Ben, Leo & Fiona


    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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