Render rust, run-off and stain removal

Orange rust and iron staining, red algae, streaks under sills and copings, and dirt marks treated with the right cleaner for each type of stain. UK costs.

In short: Render stain removal treats specific marks that a general biocide clean does not shift, including orange iron staining, red or orange algae, streaks under sills and copings, and atmospheric dirt, using a cleaner matched to the cause. It suits homes with discoloured patches on otherwise clean render. Spot treatment usually costs £100 to £300 in the UK, or less alongside a full clean.

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Not every mark on render is algae. Orange and rust-coloured staining can be iron leaching from metal fixings, aggregate or run-off, or it can be Trentepohlia, an algae that looks orange-red rather than green. Black streaks under sills are often a mix of dirt and algae carried by run-off. White crusty patches are usually efflorescence, salts brought out of the wall by moisture. Each needs a different treatment, so identifying the stain correctly comes first.

Biological stains respond to biocide, but iron staining needs a specialist rust or iron stain remover, applied carefully because strong acid cleaners can etch or lighten some renders. Efflorescence is normally brushed off dry and fades on its own once the source of moisture is dealt with. Treating the cause matters as much as the stain: a leaking gutter, a corroding bracket or a sill without a drip will put the marks straight back.

What is included

Identifying the type of stain and its likely cause
Testing the cleaner on a small area first
Biocide treatment for red, orange and black algae
Specialist iron stain remover for rust marks
Dry brushing of efflorescence and advice on moisture sources
Even rinse so treated areas blend with the rest of the wall
Pointing out the cause, such as leaking gutters or rusting fixings

Who it is for

Orange or rust-coloured marks on render
Streaks under window sills, copings and balconies
Stains around gutters, downpipes and overflow pipes
Marks around metal brackets, satellite dishes and fixings
White salt deposits on new or damp render

How much does it cost?

As a guide, spot stain treatment usually costs £100 to £300 depending on the number and size of areas, or is often added to a full render clean for a smaller additional charge. See our full cost guide.

How it works

How render rust, run-off and stain removal works, step by step

Identify the stain

The colour, pattern and location of the stain are checked to work out whether it is biological, iron, salt or dirt.

Test first

The chosen cleaner is tried on a small, discreet area to confirm it works without affecting the render colour.

Treat the marks

The right cleaner is applied to the affected areas, with dwell time to suit the stain type.

Rinse and advise

The area is rinsed evenly and you are told what is causing the stain and how to stop it returning.

Render rust, run-off and stain removal: FAQs

How can I tell if orange staining is algae or rust?

Trentepohlia algae is usually a soft orange-red film spread across damp, shaded areas and often appears alongside green algae. Rust staining tends to start at a specific point, such as a metal fixing, bracket or downpipe joint, and runs down in streaks. A biocide will clear algae, but iron staining needs a dedicated iron stain remover.

What is the white powder on my render?

That is usually efflorescence, salts carried out of the wall as moisture evaporates. It is common on new render as it dries and on walls with a moisture problem. It is normally brushed off dry rather than washed, as water can dissolve the salts back into the wall. It should fade once the wall dries out or the moisture source is fixed.

Why do streaks keep coming back under my sills?

Water running off a sill carries dirt down the wall and keeps that strip damp, which suits algae. If the sill has no drip groove, a blocked drip or too little overhang, the water runs straight down the render. Fitting or clearing the drip, or adding a sill extension, usually solves the problem after cleaning.

Can rust stains be removed completely?

Most can be removed or greatly reduced, but deep or long-standing iron staining may leave a faint mark, especially on porous or textured render. Strong acids risk etching the surface, so the gentlest effective product is used. Replacing the rusting fixing, or sealing it, stops the stain returning.