Before tiling and levelling
Screed Primer: When and Why to Prime a Screed Floor
Primer is the thin coat that decides whether tile adhesive or levelling compound actually stays bonded to the screed. Here is when it is needed and when it is not.
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A screed primer is a liquid applied to a cured screed before something is bonded to it — tile adhesive, a levelling compound, or a floor covering system. Its job is to control how quickly the screed pulls water out of the layer on top, and to give that layer a sound, consistent surface to grip.
Not every screed needs priming for every finish, and the right primer depends on the screed type. Anhydrite and cement-based screeds behave differently, and using the wrong product — or skipping a step — is a common cause of tiles debonding months later.
This guide explains when priming is needed, how the two screed types differ, and what to check before the tiler or floor layer starts. For the products themselves, see primers and bonding agents.
What a screed primer actually does
A dry screed is thirsty. Lay tile adhesive or a cement-based levelling compound straight onto it and the screed can pull the water out of the mix before it has cured properly, leaving a weak, powdery bond. Primer soaks into the surface and regulates that suction, so the layer on top cures at the rate its manufacturer intended.
Primer also binds any fine dust left after sanding, and gives a consistent surface across the whole floor — important where part of the room has been patched or repaired and would otherwise absorb at a different rate.
Priming anhydrite vs cement-based screed
Anhydrite (Gyvlon, Gypsol)
- Laitance must be sanded off before any primer goes on
- Prime with the product the adhesive or compound maker names for calcium-sulphate screeds
- Moisture limits are tighter — a CM test reading is usually asked for
- Never let a cement-based product sit on unsanded anhydrite
Cement-based (Cemfloor, sand & cement)
- No laitance sanding needed on most cement liquid screeds
- Prime with the product named for cement screeds
- Dryness is usually checked with a relative humidity reading
- Cement adhesives are chemically compatible with the screed
Residential liquid screed
- Level floors for extensions and self-builds
- Screed suitable for underfloor heating
- Build-up planned around your floor finish
Priming a screed before tiling
- 1
Confirm the floor is dry
A moisture reading comes first. Primer is not a damp-proof membrane and will not rescue a wet screed.
- 2
Sand anhydrite laitance
On anhydrite, the surface laitance is sanded off and the dust extracted before priming.
- 3
Clean the surface
Dust, debris and any contamination come off — primer bonds to whatever is on top.
- 4
Apply the primer
The primer named for the screed and the adhesive is applied as its datasheet says, including any dilution and drying time.
- 5
Tile within the window
Adhesive goes on within the primer's open time, not days later after the surface has gathered dust again.
Priming before a levelling compound
Self-levelling and latex compounds are thin, and a thin layer dries fast — which makes suction control even more important. Compound makers almost always require a primed substrate, and on a very porous screed a second coat of primer may be specified. Our floor levelling page explains where a compound skim fits versus a new screed, and specifying screed for large-format tiling covers the flatness side.
When priming goes wrong
The two classic failures are the wrong primer for the screed chemistry, and primer applied over a surface that was not ready. The first shows up as a bond that never forms; the second as tiles or compound bonded to dust or laitance rather than to the screed. Our guides to screed poured over the wrong primer and tiles debonding from screed describe what those failures look like.
The safe rule is simple: the screed maker, the primer maker and the adhesive maker should all agree on paper before anything is mixed.
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Need a floor prepared, primed and levelled?
Send the floor area and the covering planned. We'll confirm what preparation and priming the screed needs. We reply the same working day.