Does a wall need priming before tiling?

Primer sounds like an optional extra and it is doing something specific and mechanical. Cement-based adhesive needs its mixing water to hydrate and gain strength. If the wall drinks that water in the first ten minutes, the adhesive cures chalky and weak no matter how good the product was. On the opposite kind of surface, primer is doing the reverse job: giving adhesive something to grip on a surface designed to be wiped clean.

What you'll need

• the adhesive manufacturer's recommended primer
• a brush or roller
• sugar soap or degreaser for painted or previously tiled walls (optional)
• a vacuum and brush for dust

Step by step

1. Identify what the substrate actually is.Plaster skim, plasterboard, moisture-resistant board, cement backer board, sand-and-cement render, aerated block, existing tile, painted plaster — each behaves differently and the answer follows from this.
2. Test absorbency with water.Flick a few drops on. If they soak straight in, it is absorbent and needs a primer to control suction. If they bead and sit there, it is non-absorbent and needs a bonding primer or mechanical keying instead.
3. Remove dust and loose material first.Primer bonds a dusty surface to nothing very well. Vacuum, brush, and if the surface is friable, keep going until it is not.
4. On new plaster, wait until it is dry and then prime with the adhesive maker's primer.Not PVA. Acrylic tile primers are made to regulate suction without forming a water-sensitive film.
5. On plasterboard, prime and consider the wet area rules separately.Priming controls suction. It does not make the board suitable for a shower — that is a different question about board type and tanking.
6. On a painted wall, decide honestly whether the paint should stay.You are relying on the bond between paint and plaster to hold the tiles. Sound, well-adhered matt emulsion can be abraded and primed; anything glossy, flaking or vinyl silk should be stripped back.
7. On old tiles or any glazed surface, degrease first, then use a bonding primer.Soap film and body oils are the actual bonding surface if you skip this. Degrease, rinse, dry, abrade if the primer calls for it, then prime.
8. On screed or concrete, prime after checking it is dry.Priming a slab that is still giving up moisture seals the moisture in under the tiling. Do the polythene sheet test first.
9. Let the primer cure as the data sheet says before tiling.Some are touch-dry in minutes and need longer before adhesive. The sheet governs it.

Tips

• Buy the primer from the same range as the adhesive where you can — they are formulated as a system and the data sheet then covers both.
• Highly absorbent surfaces sometimes need a diluted first coat and a neat second. The product will say.
• Primer does not fix a friable or dusty surface, it only stabilises a sound one. If a wire brush takes material off in handfuls, the surface has to come off.
• Never prime over damp. Sealing damp in under tiles is how you get an area of tiles that lets go two years later.

Common mistakes

• Using general-purpose PVA under tile adhesive — PVA can soften again when it gets wet. Under a tiled wall in a bathroom that means a water-sensitive layer sitting exactly at the bond line. It is a well-known cause of tiles coming off in sheets years later, and every adhesive manufacturer specifies their own primer instead.
• Priming a dusty surface instead of cleaning it — The primer sticks the dust together and to the wall, but the dust layer itself is weak. The failure plane simply moves from the surface into the dust.
• Skipping primer on new plaster because the adhesive says it is strong — Strength is irrelevant if the plaster steals the water before the cement can hydrate. What you get is a bed that looks fine and crumbles between finger and thumb.

If it doesn't work

Questions people ask