Substrate preparation before tiling and plastering
Most tiling and plastering failures start neither in the material nor in the applicator's hand — they start in a substrate that was never prepared. The strongest adhesive on the market will part from a dusty wall, and the best plaster will crack over a surface that sucks its water out in minutes. This guide collects the checks a professional runs before opening the first bag.
The golden rule: the substrate must be sound, clean, flat and dry enough — and each of those four has a specific check and a number to measure against.
1 — Soundness and age
Tap the surface with a knife handle at scattered points. A hollow sound means a delaminated layer that must come off down to sound material. Rub the surface with your palm: if dust or loose sand comes away, the surface is weak and needs consolidating with a primer after cleaning — and if it keeps crumbling after priming, removal is safer than gambling.
Age matters because cementitious substrates shrink as they cure: new concrete needs 28 days before tiling, and cement render at least 7 days (a day per millimetre of thickness is a good base rule). Tile over a substrate that has not finished shrinking and the shrinkage happens under your tiles — the result is cracks or debonding.
2 — Cleaning: dust, oils and laitance
Dust is bonding's first enemy: a tenth-of-a-millimetre film means the adhesive bonds to dust, not to the wall. Sweep, then wipe with a damp sponge. Formwork release oils (on precast or shuttered concrete) and old paint residues need mechanical grinding or sanding — washing alone will not remove them.
Laitance is the glossy, brittle grey skin on freshly poured concrete and screeds. It looks solid but it is the weakest link in the substrate, and it must be ground off until aggregate shows. Ignoring it is the classic cause of self-levelling compounds and adhesives detaching from screeds.
3 — Flatness: the two-metre straightedge
Lay a two-metre aluminium straightedge on the surface in several directions and measure the biggest gap beneath it. For tiling: no more than 3 mm under the two-metre edge. Deeper than that and the adhesive gets spent filling hollows instead of bonding — and adhesive is not a levelling material. Bigger deviations are corrected before tiling: levelling render on walls, self-levelling compound on floors.
4 — Moisture and absorption
A cement screed must read no more than 2% CM on a carbide moisture test before tiling — 1.8% if it is a heated screed. No meter? The plastic-sheet test: tape a 50×50 cm polythene square to the floor overnight; condensation beneath it in the morning means the screed is still wet.
Test absorption with a water droplet: if it vanishes in seconds the substrate is highly absorbent and will rob the adhesive or plaster of its water before it can cure properly — that is the acrylic primer's job. If the droplet sits beaded for minutes, the surface is dense and low-absorption — that is the bonding bridge's job.
5 — Primer or bonding bridge?
The decision follows the droplet test, and getting it wrong is the single most common cause of debonding:
| Substrate | Treatment | Why |
|---|---|---|
| Cement render, blockwork, gypsum, screeds — high absorption | EF-80 acrylic primer (dilute 1:1 on very thirsty surfaces) | Regulates suction so the adhesive keeps its water while curing |
| Smooth concrete, soffits and beams cast against steel forms | EF-81 Betokontakt by coarse roller | Builds a mechanical key with quartz sand on a poreless surface |
| Sound existing tiles (tile-over-tile) | Light abrasion + bonding bridge, then a C2-class adhesive or better | A glazed face offers no bond without a key |
Drying time before you continue
Acrylic primer dries in 2–4 hours and should be covered the same day (an old, dusty primed face needs a fresh coat). The bonding bridge is left about 12 hours and worked over within two days. Never start bonding onto primer that is still tacky — tack means the film has not formed yet.
The mistakes that cost most
- Tiling concrete younger than 28 days — the shrinkage happens under the tiles.
- Leaving laitance on a screed, then wondering why the leveller let go.
- Using adhesive to fill flatness errors deeper than 3 mm instead of levelling first.
- Primer on smooth concrete (needs a bonding bridge) or a bonding bridge on a thirsty surface (needs primer).
- Tiling over old gloss paint without abrading it — the easiest debonding surface in the trade.
Frequently asked
Can I just wet the wall instead of priming?
Wetting delays the problem for minutes, then the suction returns. Primer forms a film that regulates absorption through the whole cure — water cannot do that. Wetting remains acceptable for traditional cement render on blockwork, not for thin-bed adhesives.
How do I know gypsum plaster is ready to tile over?
Fully dry (a week minimum in a ventilated room), sound under a scratch, and primed with undiluted acrylic primer — gypsum is highly absorbent and reacts with the moisture in cementitious adhesives unless the primer separates them.
Old tiles — remove them or tile over?
If they are sound with no hollow spots and the added height clears the doors, tiling over after abrasion and a bonding bridge is faster and cleaner. Any hollow area is opened and filled first. If more than a quarter of the field sounds hollow, strip it.
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Last updated: 2026-08-09