Paint or seal your surface
A tired, slippery, staining or breaking-up surface each points to a different fix.
Read the guide →The four properties that decide the choice
Two tins on a shelf can look nearly identical. Once they're on your patio, they can behave completely differently. What tells them apart is a handful of measured properties.
Slip resistance
This is how a surface behaves underfoot once it's wet, and it's tested. The result is given as a value on a scale, and a higher number means a safer surface. Ask for that value before you ask about colour.
Vapour behaviour
Often called breathability, this describes how easily moisture trapped in the wall or paving can escape through the coating. It's measured in two different units, and they run in opposite directions. One reads better the lower it goes, the other reads better the higher it goes. A figure given in one unit can't be read straight across to the other.
Rain resistance
This covers how much liquid water the coating lets through from outside, as rain. It sits on its own scale, separate from vapour behaviour. A coating can score well on one and poorly on the other, so both figures count.
Substrate compatibility
The surface underneath changes which of the other three properties decides the outcome. Dense concrete, natural stone, cement render and lime render each behave differently, and a coating suited to one can trap moisture in another. Identifying which substrate you have comes before choosing a product.
Once you know the unit, the direction and the source behind each of these four figures, two tins can be compared directly, without converting anything in your head first.
The four properties, and who defines each
Each property is measured its own way, with its own unit, so the table below keeps them separate.
| Property | Unit used | Better direction | Who defines it |
|---|---|---|---|
| Slip resistancePendulum testing is not meant to give a pass or fail result. The group's own position is that the legal requirement is simply that a surface is safe to use. | Pendulum Test Value (PTV) | Higher value means a safer surface | UK Slip Resistance Group |
| Vapour behaviourThe two units run in opposite directions, and no published conversion between them has been found, so a figure given in one cannot be read across to the other. | MNs per gram, or grams per square metre per day | Lower MNs/g, or higher g/m²/day, means more breathable | Manufacturer data sheet |
| Rain resistanceThe paired V scale runs the other way, with V1 at the vapour-open end. The standard itself is paywalled, so the numerical class boundaries aren't printed here. | W class, BS EN 1062-1 | The low end of the W scale is the rain-resistant end | BS EN 1062-1 |
| Substrate compatibilityDense concrete, natural stone, cement render and lime render each respond differently to a coating, and lime render is the one where the wrong choice can damage the wall. | No single unit | Depends on the surface, not a scale | Manufacturer data sheet, read against the substrate you have |
This sets out how each property is usually measured and by whom, not a tested result for any particular tin. Where a figure isn't printed, such as the W and V class boundaries, that's because the standard is paywalled.
Four choices before you open the tin
Whether to paint or seal, what you're coating, when the weather allows it, and what to do if it's been treated before.
Identify what you're coating
Dense concrete, natural stone, cement render and lime render each behave differently under a coating.
Check your substrate →What a sealed surface changes
A coating already on the surface changes how a new one behaves, and a warranty isn't a service life.
Find out what to ask →Start with the fault
A patio, a path or a rendered wall can go wrong in more than one way, and a tin of paint doesn't fix the same problem as a sealant. Before comparing products, it helps to name what's actually happening to the surface in front of you.
Faults that call for different answers
- A surface that's faded, patchy or just looks tired, where the finish needs covering.
- A surface that's slippery when wet, where the question is about grip underfoot.
- A surface that's staining, marking or growing algae, where the issue is what's landing on top of the coating
- A surface that's cracking, flaking or blowing, where something underneath is failing and painting over it won't hold
Each of these points to a different fix, and a tin picked by category alone can leave the real fault untouched. A slippery flag needs a figure for grip. A render that's blowing needs the substrate identified first, because a lime render behaves differently from cement render or dense concrete and the wrong coating on it damages the wall rather than just the finish. Paving that's merely faded is a more straightforward job, covered in more detail where the question is simply whether paving slabs can be painted at all.
Working from the fault is what narrows two tins down to the one that actually answers your problem.