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Surface Preparation

Plaster skimmed plaster

Plastered walls are one of the most common tiling substrates, and also one of the easiest to get wrong. Plaster can vary a lot in strength and suction (how quickly it absorbs water). If the wall is dusty, damp, weak, sealed, or too smooth, tiles often sound hollow later or debond completely. This guide helps you check the plaster properly and prep it the right way, without guesswork.

Minimum requirements before you start

Plaster must be cured, dry, and stable before any priming or tiling.

Curing

  • Most cement plaster should cure for at least 7 days. Thicker builds or polymer-modified plasters may need up to 14 days. If the plaster still feels cold or damp, it is not ready.

Dryness

  • Only tile once the wall is properly dry (as a guide: moisture below 5%).

Stability

  • Do not tile over movement, bulging patches, loose plaster, or active cracks. Fix the substrate first.

Quick checks that tell you what prep you need

Two quick tests prevent most plaster failures.

Strength check (rub test)

  • Rub the wall with your hand or a clean cloth. If it powders easily, the surface is weak or chalky and needs extra prep and correct priming. If it stays firm, you are in a good place.

Absorption check (water drop test)

  • Drop a little water onto the plaster.
  • If it soaks in immediately, the plaster is very porous and will pull moisture from the adhesive unless you prime correctly.
  • If it absorbs slowly (around 10 seconds or more), it is normal suction.

Quick checks that tell you what prep you need

Two quick tests prevent most plaster failures.

Strength check (rub test)

  • Rub the wall with your hand or a clean cloth. If it powders easily, the surface is weak or chalky and needs extra prep and correct priming. If it stays firm, you are in a good place.

Absorption check (water drop test)

  • Drop a little water onto the plaster.
  • If it soaks in immediately, the plaster is very porous and will pull moisture from the adhesive unless you prime correctly.
  • If it absorbs slowly (around 10 seconds or more), it is normal suction.

Preparation steps

Start with the basics, then choose the priming method that matches your plaster.

Light sanding (almost always worth doing)

  • Even good plaster can have a fragile surface film. Lightly sand with 60 to 120 grit to remove laitance (the soft top layer), knock off weak material, and create a better key. You are not trying to grind the wall, just dull and strengthen the surface.

Clean properly

  • After sanding, remove all dust. Brush down, vacuum if available, then wipe with a lightly damp cloth (do not soak the plaster). Let the surface dry again fully. Dust left behind is one of the biggest causes of poor bond.

Remove contaminants

  • Plaster often gets contaminated on site (paint overspray, bonding liquids, wax or polish, hand oils, construction dirt). If anything is sitting on the surface, the adhesive cannot bond to the plaster, it bonds to the contaminant. Sand or abrade and clean until the surface is sound.

Priming plaster

Priming prevents flash drying and helps ensure long-term adhesion. Choose the method based on what your tests showed.

Normal or slightly porous plaster

  • Apply a uniform coat of Kwik-Key diluted 1:1 with water (always follow the product label for dilution guidance). Use a block brush and work it into the surface evenly.

Very porous or powdery plaster

  • Use two coats of neat Kwik-Key, allowing the first coat to dry before applying the second. This seals suction and stabilises weak surface strength so the adhesive does not dry out too fast.

Painted, sealed, or water-beading plaster

  • Plaster often gets contaminated on site (paint overspray, bonding liquids, wax or polish, hand oils, construction dirt). If anything is sitting on the surface, the adhesive cannot bond to the plaster, it bonds to the contaminant. Sand or abrade and clean until the surface is sound.
  • If water beads or runs off, treat it as a non-porous surface. Sand aggressively until the finish is dull and opened up, clean thoroughly, then apply a Kwik-Key slurry coat to create a cementitious bonding surface.

For the exact slurry method, refer to the article: 

Kwik-Key Slurry Guide

Final checks before tiling

  • Before you start spreading adhesive, the plaster should feel firm (no powdering), clean, dry, and even (no bumps, hollows, or loose patches). If you are unsure, do not guess. It is far cheaper to prime correctly than to repair a failed tile installation later.

Coverage rules (bond quality)

  • Aim for at least 80% coverage in dry areas.
  • Aim for about 95% coverage in wet areas and splash zones.
  • Poor coverage is one of the main reasons tiles sound hollow later, especially on walls.

Movement and gaps

  • Always leave a small perimeter gap and use a flexible sealant where a tiled wall meets corners, ceilings, floors, countertops, baths, or other materials. Do not fill these changes of plane with grout.

Never tile over these plaster conditions

  • Stop and fix first if you see any of the following: uncured or damp plaster, powdery or chalky surfaces, painted or sealed plaster that has not been sanded and keyed, moisture ingress or mould, bulging or hollow patches, and cracks that have not been repaired and stabilised.