Start with what you're actually coating
A coating is formulated for a particular kind of surface, and a surface is not one thing. Dense concrete, natural stone, cement render and lime render each take water, salts and the product itself differently, so a tin chosen for one can sit badly on another.
Working out which of these you have comes before choosing a product. It decides what you ask for at the counter, what you rule out straight away, and in some cases what you should not do at all.
What's at stake in getting it wrong
On a concrete path or a stone step, the wrong product mostly costs you the job itself: poor adhesion, early flaking, a surface that needs stripping back sooner than it should. On a rendered wall, particularly an older or solid-walled one, the consequence runs further. A coating that stops the wall breathing as it needs to can trap moisture inside the render, and the damage then sits in the wall behind it.
That is why the substrate gets identified before anything else, and why a lime render is treated differently from a cement one.
Dense concrete, natural stone, cement render and lime render
Find the substrate you are working on, then check the property it is judged on and the figure to ask a supplier for.
| Substrate | What dominates | Figure to ask for | Risk of getting it wrong |
|---|---|---|---|
| Dense concrete | Low porosity, so water tends to sit on the surface | Liquid water permeability (W class) and a slip resistance figure for the finish | A coating that cannot shed surface water can blister or peel |
| Natural stone | Porosity varies by the specific stone, so no single figure covers all of them | Which stone you actually have, then the vapour resistance figure for that stone | Treating every stone as one substrate can hide a stone that needed a breathable coating |
| Cement render | Moderate porosity, built to let some moisture through but less than a lime render | Vapour resistance figure and the V class the coating is sold against | A coating that is too closed can trap moisture behind the render and crack it |
| Lime render | High vapour permeability, the render is designed to let the wall breathe | Vapour resistance in MNs per gram, with a figure at the lower end | A vapour-closed coating traps moisture in the wall itself |
This sets out general substrate behaviour, and it does not cover every stone type or render mix you might find on an older property.
How can you tell lime render from cement render?
A render wall does not come with a label on it, and the two common types behave very differently once a coating goes on. A few checks by eye and by hand will usually tell you which one you are looking at.
Look at the age and the texture
Lime render was the standard choice on solid-walled buildings before the First World War, and it tends to look it. The surface is often slightly uneven, with a warmer off-white or cream colour. Cement render became common afterwards and reads as harder and smoother, laid in a tighter, more even coat.
Check how it has cracked
Lime render moves with the wall as it expands and contracts, so its cracks tend to be fine and spread across the surface in a network, sometimes called crazing. Cement render is more rigid. When it cracks, the cracks are usually fewer, straighter and wider, often running in a single line.
Try a scratch test
Draw a key or a flat screwdriver lightly across an inconspicuous patch. Lime render is soft enough to mark or crumble a little under light pressure. Cement render resists the same scratch and will usually only mark under a lot more force.
Watch how it takes water
Splash a small amount of water onto the wall and watch what happens. Lime render tends to darken quickly and spread the patch as it soaks in, because it is more open to moisture. Cement render tends to bead or sit on the surface longer before it is drawn in, because it is denser.
None of these checks is conclusive on its own, and an older wall can carry patches of both renders side by side. Where the checks point in different directions, treat the wall as uncertain and ask a supplier the question before choosing a coating.
Why lime render needs its own answer
A lime render that's painted with the wrong coating doesn't just look worse. Moisture that would normally pass through the wall gets trapped behind the film, and over time that can soften the render, push it off the wall, or carry damp further into the structure than it would otherwise go. On a solid-walled or older property, the render is often doing a job a modern cavity wall doesn't need it to do, which is why a coating choice that's fine on dense concrete can be the wrong one here.
Why vapour behaviour decides this
The property that matters on a lime render is how much water vapour the coating lets pass, stated as either vapour resistance in MNs per gram, where a lower number means more breathable, or water vapour transmission rate in grams per square metre per day, where a higher number means more breathable. A coating with the wrong figure on this scale can hold moisture inside the wall. Checking that figure before buying is the single step most likely to prevent damage here, and it's covered in full on the vapour behaviour reference.
Confirming what you're dealing with
Lime render and cement render can look similar from a distance but behave very differently once coated. Before choosing a product, it's worth working out which one you actually have.
- Check how the render feels and sounds when tapped
- Look at the age and build of the property, since older walls are more likely to be lime
- Check for an existing coating and what state it's in
Once you know which substrate you're working with, the substrate identification guide sets out the property to ask for on each one, and what tends to go wrong when it's ignored.