Why Porosity and Water Absorption Decide Your Softwash Results

Mark Cave

Picture this. Same mix, same afternoon, three surfaces. A dry timber deck, a dry sandstone patio and a uPVC fascia. Treat all three the same way and one gets damaged, one barely gets cleaned, and one just watches your chemical run off onto the lawn. Same bucket, three different problems. And they all come down to something a lot of people never think about: what the surface does with water.

Mark Cave

I'm Mark Cave from SoftWash UK, and today we're talking about water absorption and porosity. If you've listened to the episodes on why contact beats concentration, or the test patch paradox, this is the physical reason sitting underneath both. How long your chemistry really works isn't decided by what's in the drum. It's decided by the surface you're standing in front of.

Mark Cave

Let's separate two ideas that get muddled. Porosity is how much open space is inside the material. Think of the number of tiny holes. Concrete, sandstone, clay tiles, monocouche render, timber, they all have them. Absorption rate is how hungry those holes are. It's the capillary pull, the way a dry pore draws liquid inward. A surface can have plenty of pores, but when it's already full of water, that pull is switched off. Dry it out in warm UK weather and it switches right back on.

Mark Cave

Now the opposite end. uPVC, glazed tile, powder coated cladding, dense porcelain. These don't drink anything. Your chemistry sits on top, and it behaves like water on a waxed car. It sheets, it beads, it runs. So on those surfaces you're not worrying about absorption at all. You're worrying about cling. That's where a proper surfactant earns its place, something like Clever Wash, because it lowers surface tension and helps the solution spread and hang on instead of racing to the ground. More chemical strength won't fix a solution that won't stay put.

Mark Cave

Now timber, because this is where the thirsty surface does the most harm. A deck that's been baking for a few dry days is gasping for a drink. Put sodium hypochlorite on it, and those tubular fibres pull the chemical deep into the wood. Once it's in there, you can't rinse it back out. It keeps oxidising the lignin and the natural oils for days. The result is fibre damage, that furred, sunburn peeling look, and a permanent silvery grey bleaching.

Mark Cave

The fix costs nothing. Give the wood a drink first. Soak it with clean water, ideally for around an hour if you can. You're filling up those fibres so they have no room left to take your chemistry. Then a light mix, about one percent sodium hypochlorite with surfactant, stays on the outer layer. And that outer layer is exactly where the algae lives. Let it dwell for a short time, rinse it off completely, and the structure of the wood is protected.

Mark Cave

Here's where people get caught out. They hear that, and they decide to soak everything. Stone, block paving, concrete, the lot. And on black spot lichen, that backfires. Lichen builds a protective biofilm, and its roots anchor down inside the pores. To kill it, your biocide has to get in there. If you've pre soaked the stone, you've filled those pores with clean water. Your chemistry hits that water at the point of contact and dilutes instantly. It sits on top of a barrier, and never reaches the roots.

Mark Cave

So on dry masonry, you actually want some of that capillary pull working for you, drawing the active chemistry down into the pores. Wood needs to be full. Stone needs to be ready to drink. That's the contrast. And the plants next to it? You protect them separately, wet the soil and the planting, keep control of overspray, and leave the stone itself alone.

Mark Cave

That brings us to dwell time, and I want to be strict about what it means. Dwell time is wet contact time. That's all. Once your solution dries, the cleaning stops. What you're left with is crystallised salts, tiger striping, ghost marks, and sometimes surface etching.

Mark Cave

Porous surfaces make that harder, because they pull moisture inward and strip the film off the surface, even when the air isn't especially dry. Add sun on a south facing wall of K Rend or brickwork, and both absorption and evaporation speed up together. A common mistake is to spray a whole elevation, then walk back to find the top section already dull and dry. Work in manageable sections. Track the sun. And if a porous section starts to flash dry, lightly mist it with solution to keep it wet. Mist it, don't flood it, because flooding just dilutes and runs. Your surfactant dose matters here too, so the solution spreads evenly before the surface can steal it.

Mark Cave

Now rinsing, and this is where the chemistry you chose changes the rule. With sodium hypochlorite on a highly porous surface, you want a careful, thorough rinse. You're flushing out dissolved biological soil and chemical salts from the pore network. Leave those behind, and you risk problems like efflorescence, or damage under the surface that doesn't show until later.

Mark Cave

A slow acting quaternary biocide, like Soft Wash Pro 50, is a different job. If you're applying it as a residual treatment, the whole point is for the surface to take it in and hold it in the pores, then weather off gradually over weeks or months. Rinse it straight away and you've undone the treatment. So rinse or don't rinse isn't a habit. It's a decision, and it depends on the product label, the chemistry and the substrate.

Mark Cave

So what do you do on Monday morning? Before you switch a pump on, look at the surface. What is it made of? Is it dry or damp? How old and weathered is it? Is it in full sun? Do a small test patch, watch how fast it drinks and how fast it dries, and let that decide your pre wetting, your mix, your sections and your rinse. Guessing strengths and soaking everything is how a splash and dash operator works. Reading the surface is how a technician works.

Mark Cave

Podcasts like this are a great start, but being able to judge porosity, choose the chemistry, set up your equipment and stay safe and compliant takes a deeper understanding. That's exactly why the CPD Certified Softwash Training Course exists. It's online, so you learn at your own pace, and it's built to help you avoid costly mistakes. 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

Here's your challenge for the week. On your next three jobs, before you mix anything, flick a little water onto the surface and watch what it does. Does it bead, soak straight in, or sit and darken? That ten second check tells you more than most people realise. Take care, stay safe, and I'll catch you in the next one.