Diagnosing Red Stains on Render: Algae vs Rust

Mark Cave

A red stain on a render wall looks like one problem. It's usually two, and they need completely different treatment. I see contractors get this wrong all the time, and it costs them chemical, time and sometimes the surface itself.

Mark Cave

I'm Mark Cave from SoftWash UK. Today is a contractor coaching session on red, orange and salmon staining on modern render. Think K Rend, monocouche, silicone and acrylic finishes. A while back we talked about cleaning monocouche safely, the chemistry, the cling and avoiding wand marks. Today is the diagnosis step that comes before any of that.

Mark Cave

Start with the most common cause on UK render. It's biological. Red, orange or salmon algae, and one organism often linked to it is Trentepohlia. It's an orange pigmented green algae, and the colour comes from carotenoid pigments. Those pigments help it tolerate sunlight, which is why it can sit happily on a wall that gets a fair bit of daylight.

Mark Cave

So how does it look on site? Broad curtains or diffuse patches. Think of water running down from beneath coping stones or sills. Think shaded, north facing elevations, walls tucked behind trees or fences, areas below leaking gutters. It follows moisture. If the stain is soft edged and spread across a damp, shaded area, your first thought should be biology.

Mark Cave

Now the other culprit. Iron oxide. Rust. That's inorganic, and it looks different. You get distinct, linear vertical tails with a clear starting point. A corroding screw. A sign bracket. A metal corner bead breaking down under the finish. Even a satellite dish fixing. Find the metal and you've usually found the stain.

Mark Cave

Colour alone will fool you. So walk back from the wall and read the building. Is there a source point? Does the stain spread across a damp zone, or run down in a line? Is it only the north side that's affected? Only the north side points to biology. A tail under a bracket points to rust. And yes, you'll often find both on the same elevation. Algae across the wall, and rust streaks under one corroded fixing.

Mark Cave

Here's why this matters. The chemistry is completely different. Sodium hypochlorite and DDAC biocides will not remove iron oxide. Worse, you may end up setting a mineral stain. So you've wasted product, and the stain is still there.

Mark Cave

Rust needs an acidic reducing or chelating agent, and oxalic acid is the classic example. A dedicated rust remover, tested first and used to the label. But flip it around. Put a rust remover on red algae and nothing useful happens. You've wasted chemical, and if you go chasing the stain with stronger and stronger mixes, you can etch the aggregate. Once render is etched, you can't undo it.

Mark Cave

Knowledge comes before products. That's the whole point. Anyone can buy a drum of chemical. Reading the wall is the skill.

Mark Cave

Let's talk workflow. Before you touch anything, survey the source points. Look at fixings, beads, brackets, copings, gutters and sills. If a leaking gutter or a corroding fixing is feeding the stain, tell the customer, because otherwise it comes back.

Mark Cave

Then do a discreet test patch. Somewhere out of the way, with the dilution you plan to use, and watch what happens. That patch tells you if you've diagnosed it correctly, and it protects you if the customer asks questions later.

Mark Cave

On vertical elevations, apply from the bottom up. That stops clean streaks tracking down through dirty areas, which is a classic way to leave a wall looking striped. Then rinse top down, with low pressure. Render is not the surface to blast, and you don't want to drive water behind the system.

Mark Cave

For lasting control of red algae on porous render, a residual biocide makes sense. Soft Wash Pro 50 is a 50% DDAC concentrate, HSE number 10245, and the guide dilutions are 1% for maintenance cleans and lighter contamination, and 2% for heavier first time cleans where suitable. That's a guide, not a rule. Assess the surface, the weather and the runoff first. Some staining takes time to weather away after treatment, so set that expectation with the customer.

Mark Cave

And contain your runoff. Protect plants, ponds and drains. Always read the label and the safety data sheet. Professional work means controlling where the chemical goes, not just where it works.

Mark Cave

Podcasts like this are a great starting point, but diagnosis, chemical choice, dilution, safety, COSHH and pricing all need deeper understanding. That's what the CPD Certified Softwash Training Course at softwashing.uk is built for, from years of real world experience. It's online, at your own pace, and completing it lets you become an Approved Softwash Applicator, with a directory listing and a place in the National Association of Softwash community.

Mark Cave

Next time you see a red stain, ask one question before you open a drum. Is there a metal source, or is there moisture? Answer that and you're halfway there. Take care, and I'll catch you in the next one.