Dilution Ratios vs Applied Percentages in Softwashing

Mark Cave

Ask ten exterior cleaners what a one to five mix is, and I'd bet you get at least two different answers about what's actually going onto the wall. That's a problem. Because when the slang is fuzzy, the chemistry is fuzzy, and when the chemistry is fuzzy, somebody's render, somebody's margin, or both, are being gambled with.

Mark Cave

I'm Mark Cave from SoftWash UK, and today is a straight talking one about a mistake I see constantly. Contractors confuse part ratios with the actual percentage of active chemical they're applying.

Mark Cave

Let's start with the stock. In the UK, commercial sodium hypochlorite typically arrives at 14 to 15 percent available chlorine. We supply it in 20 litre boxes, four 5 litre containers to a box. So let's use 15 percent as our working figure.

Mark Cave

Now, somebody says, I run a one to five. What does that mean? One part sodium hypochlorite, five parts water. Count them up. That's six parts in total. Not five. Six. And the strength you're applying is the stock strength divided by the total parts. So 15 percent divided by 6 gives you 2.5 percent.

Mark Cave

Here's where it goes wrong. A lot of contractors divide by the water parts alone. They take 15, divide by 5, and tell themselves they're putting on 3 percent. They're not. They're putting on 2.5. That's a gap of half a percent, and it feeds straight into your chemical budgeting, your job pricing, and your confidence about what the surface can take.

Mark Cave

Same thing with a one to six. That's seven parts in total. 15 divided by 7 is 2.14 percent. If you divide by six, you think you're at 2.5. So you've got the same slang, a different answer, and nobody on the van noticing.

Mark Cave

And this connects back to our earlier episode, Softwashing Chemistry: Why Contact Beats Concentration. We said dwell time and contact are king. True. But you can't control contact until you know exactly what percentage is sitting on the substrate. Contact with an unknown strength isn't control. It's hope.

Mark Cave

So let's do it properly, with real batches. Say you want 100 litres of 2.14 percent. That's a one to six, seven parts. 100 divided by 7 is about 14.3 litres of sodium hypochlorite. Top it up with about 85.7 litres of water. Roughly 14 and 86 on the measuring marks.

Mark Cave

Now 125 litres at 2.5 percent. That's a one to five, six parts. 125 divided by 6 is about 20.8 litres of sodium hypochlorite. Many people round that to 20 litres of SH and 105 of water, which lands you just a touch under 2.5. That's fine, as long as you know you've rounded and you're not pretending it's exact. The habit that matters is working from total parts, then writing the number down.

Mark Cave

Then there's the surfactant. This is where a lot of people try to fix a wetting problem with extra strength. Don't. Our Clever Wash Softwash Surfactant goes in at 200 to 1. That's 500 millilitres per 100 litres of mix, or 625 millilitres for a 125 litre batch. What it does is reduce the surface tension of the water, so the solution clings rather than sheeting straight off vertical render. Better cling means better dwell time, and better dwell time means a lower strength can do the oxidising work without brute force concentration.

Mark Cave

So what strengths are we actually aiming for? I'll give you the starting points I'd work from, and I'll be clear, these are guidance, not gospel. Every surface gets a test patch first.

Mark Cave

Modern thin coat and monocouche renders, K Rend type finishes, generally sit at around 1 to 2 percent. I'd rarely go beyond 2.5. Push harder and you risk bleaching pigment, drawing salts, and shocking the surface. Heavy organic flatwork, concrete and black spot, typically 2.5 to 3.5 percent, and only occasionally up to 3.75 for stubborn lichen. Degraded timber, keep it strictly under 1.5 percent, and pre wet it, or you'll be looking at raised grain and a very unhappy customer.

Mark Cave

Why does it work at those levels? Sodium hypochlorite oxidises organic contamination. Algae, mould, moss, lichen. It breaks that biological growth down at a cellular level. It isn't just bleaching the colour out. But it needs contact time to do it, which brings us full circle to getting the percentage right and the cling right.

Mark Cave

Now the safety side, because it matters. Never mix acids with sodium hypochlorite. You get toxic chlorine gas. Keep your products segregated, in the van and in the yard. When you mix, add the chemical to the water, never water onto chemical, because you don't want it splashing at your face. Wear your gloves and eye protection, mix somewhere ventilated, and stir it through.

Mark Cave

On the regulatory side, I'm told under the Control of Explosives Precursors and Poisons Regulations 2023, sodium hypochlorite over 6 percent available chlorine is treated as a reportable poison. So expect purchasing validation, and keep your storage secure. Check the current guidance for your own circumstances, because the rules do change.

Mark Cave

And one more thing that catches people out. Raw sodium hypochlorite breaks down over time. Heat, sunlight and age turn it, ultimately, into salt water and oxygen. So that drum that's been sitting in a warm shed since last summer is not the 15 percent you bought. If you're calculating from 15 on the label, but the drum is at 11, every one of your lovely sums is wrong. Rotate your stock, store it cool and out of the sun, and test it if you're unsure.

Mark Cave

So, what can you do this week? Stop saying one to five without saying the percentage. Write the applied strength on every batch. Work from total parts. Check your stock is in date and stored properly. And do a test patch before you commit to a surface.

Mark Cave

Podcasts like this are a great place to learn, but if you're serious about offering softwashing professionally, the structured route is the SoftWash UK CPD Certified Softwash Training Course. It's online, so you learn at your own pace, and it goes much deeper into chemical selection, surface identification, safety and pricing. Complete it and you can become an Approved Softwash Applicator, get listed in the directory, and join the National Association of Softwash community. Have a look at softwashing.uk.

Mark Cave

Knowledge comes before product. Always has. Thanks for listening, and I'll catch you in the next one.