The slab decides the answer
Paving slabs can be painted or sealed. Most of the surfaces people have in mind, whether dense concrete, natural stone or cast slabs, will take a coating of some kind.
What decides whether that coating holds and whether it leaves the slab safer or more slippery once wet is what the slab is made of, how it has weathered and what is already sitting on it. A slab fresh from a supplier behaves differently from one that has been down for years and picked up moss, algae or an earlier coat nobody can identify.
The slab itself gives the answer before any tin does. Working out what it is, what state it is in and how it behaves once wet and once coated puts you in a position to choose the right product.
What kind of slab are you actually working with?
Before any paint or sealant goes anywhere near a patio, the slab itself needs identifying. A dense, precast concrete slab and a natural stone slab can look similar enough on a damp afternoon, but they are different materials, and a coating chosen for one can cause problems on the other.
Precast concrete versus natural stone
Pressed concrete slabs are manufactured in a mould, so the surface tends to be uniform: the same texture, the same colour, repeated across every slab in the pack. Natural stone, whether sandstone, limestone or slate, varies piece to piece. You'll see grain, veining, shell fragments or colour banding that no two slabs share exactly. That variation is a sign of a more open, more porous structure than cast concrete usually has.
Why the difference changes the answer
A coating that sits well on a dense concrete surface is doing its job on a material that lets relatively little moisture pass through it. Lay the same coating on a more porous natural stone and the moisture in the ground beneath the slab still needs somewhere to go. If the coating blocks that path, water collects under the film. On a stone that is vulnerable to frost, that trapped moisture is what causes damage.
Working out which you have comes first, before looking at any tin. Check whether the slabs are identical across the patio, which points to concrete, or whether each one has its own pattern and grain, which points to natural stone. If you're not certain, a cut edge or a broken corner usually shows the structure more clearly than the surface does.
Will a painted slab be slippery when wet?
A paving slab gets most of its grip from the texture already in its surface, and wet weather removes a good part of that before any coating goes on. Put a layer of paint or sealant over the top and you change that surface again, sometimes for the better and sometimes for the worse. Colour and sheen won't tell you which way it has gone. The only way to know is to ask whether the coating has been tested wet, on a surface like yours.
The test to ask a supplier for
The test that measures wet grip is the pendulum test, carried out to BS 7976-2. A weighted rubber pad is swung across the wet surface, and the drag it meets gives a figure for how much grip is on offer underfoot. A supplier who can produce a result from this test for the product you're considering has given you something to compare. One who can't has given you a colour chart and little else.
It's worth putting the same question to a sealant as to a paint, and to a smooth finish as to a textured one. The surface a coating actually leaves behind doesn't always match what the tin implies.
What to check before you choose a coating
Two tins can look almost identical on a shelf and still behave completely differently once they are on a slab outside. What separates them is not colour or price. It comes down to four measured properties: what the surface is made of, how slip-resistant the finish is when wet, how it handles moisture vapour, and how well it keeps rain out.
What the surface is made of
Dense concrete, natural stone, cement render and lime render do not all respond the same way to a coating. A film that sits well on dense concrete can trap moisture behind it on a rendered wall. A separate guide covers how to tell one substrate from another.
How slip-resistant it is when wet
A painted slab can end up more slippery when wet than an unpainted one, unless the coating has been tested for it. The test to ask a manufacturer about is the pendulum test, reported as a Pendulum Test Value. The bands published by the UK Slip Resistance Group, and the limits the group places on how that figure can be used, are set out on the slip-resistance page.
How it handles moisture vapour
Breathability is reported in one of two ways. Vapour resistance is given in MNs per gram, where a lower number means a more breathable coating, while water vapour transmission rate is given in grams per square metre per day, where a higher number means more breathable. The two scales run in opposite directions and measure different things, and a page on vapour behaviour works through both before comparing any figures.
How well it keeps rain out
Under BS EN 1062-1, a coating gets a W class for resistance to liquid water and a V class for openness to vapour, and the useful end of one scale is the opposite end to the useful end of the other. A reference page on this standard sets out which direction is which, and states where the published class boundaries are not available without buying the standard itself.