Render cleaning without overdoing the pressure
The most expensive thing we see on a render elevation is not dirt. It is the last person who cleaned it.
What the damage looks like
On painted render, a lifted edge somewhere around waist height where the operator stood, and a slightly different sheen in a band across the wall. On K-Rend and other silicone renders, a faint scoured stripe that only shows in raking light — until the elevation re-soils and the stripe stays clean, at which point it is the first thing anyone sees. On pebbledash, bare patches where the aggregate has gone.
None of it is dramatic on the day. All of it is permanent.
Why it happens
Because pressure produces an instant result and chemistry does not. A wand at close range clears a metre of wall in seconds and the customer standing behind you is visibly pleased. A biocide looks like almost nothing for a fortnight. If you are being judged on the moment you pack up, pressure wins every time.
That is the whole problem with how this trade is sold. The method that protects the building is the method that looks worst on the day.
What we do instead
Identify the render first — silicone, monocouche, cement, lime or painted — because they are not one material with one answer. Then a biocide at about garden-hose pressure, applied top-down, with the dwell time the product actually needs rather than the time the schedule allows. Then a gentle rinse. Then we tell the customer, before we leave, that the elevation will look partly done for the next few weeks and finished after that.
Setting that expectation is not a caveat. It is the job. The customers who are unhappy at the four-week mark are always the ones nobody warned.
The one exception worth knowing
Not everything on a render wall is organic. Iron staining runs in streaks from a fixing, railing or planter, and no amount of biocide or dwell time will touch it. It needs a specific iron treatment applied carefully, because the same chemistry that lifts the stain can etch what is underneath. If somebody quotes you a standard soft wash for a rust run, they have not looked properly.
More on what the colour of the staining tells you · K-Rend and silicone render cleaning