Uneven wall before plastering or tiling: what to use

The wall is not flat. Now what.

Blockwork and old plaster are rarely flat enough to go straight to a finish coat or tile adhesive. Hollows behind a straight edge, a wall that is out of plumb at the top, or patches where old plaster has been hacked off all need building up first. The mistake we see most often on site is reaching for whatever is in the van and applying it too thick in one go, or using a tiling product where a plastering product was needed. The two jobs use different materials, and mixing them up costs time later.

Two different finishes, two different products

If the wall is going to be skimmed and painted, you need a plaster that bonds to blockwork or old plaster and can be built up in a single coat without slumping. If the wall is going to be tiled, you need a mortar that gives tile adhesive a flat, stable base to grip, not a gypsum surface that was never meant to sit under tile.

Cekol GS-100 Gypsum Plaster 1-10mm is the base coat for the first case. It is applied by hand or machine, builds up hollows and out-of-plumb areas in one coat, and is designed to be skimmed over afterwards with a finish plaster such as Cekol GS-200 Gypsum Finish Skim Coat. It is not a tiling substrate.

Cekol ZW-04 Levelling Mortar 3-50mm is the one for the second case. It is a cement-based levelling mortar rather than a gypsum plaster, so it stands up to being tiled directly, and it covers a wider depth range, which matters on a wall or floor that is badly out.

Product	Base	Typical use	Depth range
Cekol GS-100 Gypsum Plaster	Gypsum	Building up walls before a skim finish and paint	1-10mm
Cekol ZW-04 Levelling Mortar	Cement	Levelling walls or floors before tile adhesive	3-50mm

Check the datasheet for exact mixing ratios and coverage per bag, since these depend on the depth you are applying and how much water the substrate pulls out of the mix.

Prime before either one goes on

Both products need the substrate primed first, and which primer depends on what is underneath. A dusty or absorbent block wall needs a deep penetrating primer to stop it pulling the water out of the plaster or mortar too fast, which is what causes it to crack or fail to key. We stock Cekol DL-80 Deep Penetrating Primer for that. Where the surface is smooth, dense, or was previously painted, for example old sand and cement render or a smooth concrete lintel, a bonding primer with sand in it gives the new material something to grip. That is what Cekol Quartz Sand Bonding Primer 5kg is for. Using the wrong one, or skipping it altogether, is the single biggest reason a base coat plaster or levelling mortar debonds later.

Build it up in stages, not one thick pass

Both products have a maximum thickness per coat stated on the bag. On a wall that is badly out of plumb, that means two passes rather than one thick one, with the first coat allowed to firm up before the second goes on. Trying to push 30mm on in one go with a product rated for less per coat is how you end up with slumping, cracking, or a coat that never fully dries through before the finish goes over it.

Getting the surface right for what comes next

Once the base coat plaster has gone off, key or scratch the surface lightly before skimming, so the finish coat has something to bite into rather than sitting on a smooth, closed face. Once the levelling mortar has cured, check it is flat with a straight edge before tiling, since tile adhesive is not designed to correct a wall that is still out of true, only to bed the tile to a surface that already is.

Browse the full Joint Fillers & Wall Plasters range for base coats, finish coats and jointing products, or the Tile Adhesives & Mortars range if the wall is being tiled. For tiling, once the wall is level, Sopro S-Flex Gel Tile Adhesive Grey is a solid highly elastic adhesive to follow it with.

Send us the wall dimensions

If you are not sure how out of plumb the wall is or which product it needs, send us the dimensions and roughly how uneven it is, and we will work out how many bags to order and which primer to use underneath. In-stock orders go out within 7 working days, with free delivery in Ballincollig and Cork City. Collection is by appointment only.

Call 085 868 8459 or email sales@all4building.ie and we will sort the materials list out with you.