Skim Coating: Base and Finishing Layers

Base and finishing fillers do different jobs: one corrects the surface, the other creates a smooth ground for paint. Differences, sequence and inspection.

The wall has been plastered, but it is not ready for paint yet. Plaster gives the geometry — it makes the wall plumb and flat — but its surface is grainy, with trowel marks and small pits. The stage that sits between that surface and the paint is skim coating, and it is normally done with two different materials rather than one.

What the base coat does

The job of the base layer is to fill what the plaster left behind: trowel marks, shallow dips, uneven corners and the joints between plasterboard sheets. This filler has a relatively coarse grain, so it tolerates thicker application with less risk of cracking and builds the wall up quickly.

After the base coat the surface is still not perfectly smooth; you can feel a fine roughness. That is expected. Its purpose is to shape the wall and give the finishing coat an even ground.

Mesh is often used at this stage. Fibreglass mesh helps stop hairline cracks from reappearing on old walls and reinforces joints in plasterboard. The mesh must be embedded inside the filler, never left showing on the surface.

The finishing coat: thin layer, smooth result

Finishing filler has a very fine grain. It is drawn on in thin coats and, after sanding, leaves an almost glass-smooth surface. Applying it thickly is a mistake: a fine material in a heavy layer can crack or shrink as it dries.

The quality of the finishing coat shows most clearly under raking light. Sunlight falling across the wall from a window, or a fitting mounted close to the wall, reveals small ripples, sanding marks and transitions. A matt paint can hide some minor defects, while satin and gloss finishes are far less forgiving of an untidy finishing coat.

Layer sequence and the logic behind each step

A typical sequence looks like this:

• dust removal and priming of the plastered surface;
• mesh where needed, then the base filler;
• once dry, coarse sanding and dust removal;
• primer again;
• one or two thin finishing coats;
• fine sanding, dust removal and a final primer before painting.

Priming between coats can look like an unnecessary step, but it evens out adhesion for the next layer and stops the surface from absorbing moisture unevenly. Drying time depends on the manufacturer’s instructions, the thickness of the coat and the temperature and humidity in the room. A layer covered too soon tends to show up later as patches or cracks under the paint.

Gypsum or polymer-based filler

In dry rooms gypsum-based fillers are common: they are easy to work and sand well. For bathrooms, around a kitchen splashback or in other humid areas, moisture-resistant cement or polymer-based materials are chosen instead. Ready-mixed polymer pastes are convenient for finishing work because they remove the risk of mixing errors.

One rule is worth keeping: staying within one system is more reliable. Mixing base filler, finishing filler and primer from different manufacturers sometimes leads to adhesion problems.

Where defects that appear after painting come from

Most problems that become visible after painting start at the skim-coating stage. If the square pattern of mesh shows on the wall, the mesh was left too close to the surface and not enough filler was drawn over it. Small round pinholes usually come from air bubbles trapped while mixing, or from a finishing coat applied too thickly.

A thin crack running along a joint often means plasterboard joints were not reinforced with tape or mesh. Differences in sheen across the wall point to sanded and unsanded areas absorbing paint differently — in other words, the final coat of primer was skipped.

These defects can be corrected after painting, but it usually takes local filling, sanding, priming and repainting the entire wall. An hour saved at the start ends up costing several days. So it pays not to rush the breaks between coats and to check each coat under a light.

What to check at handover

The moment after skim coating and before painting is the cheapest point to fix anything. Every defect will look more pronounced once paint is on.

• Shine a strong torch along the wall from the side and mark any shadow-casting waves.
• Look at the lines of corners and window reveals; edges should not be rounded off or chipped.
• Run your hand over the surface: there should be no unsanded drips or grit.
• Make sure the dust has been fully removed, otherwise paint will not bond properly.

In its apartment and office renovations Zaferoğlu İnşaat builds up wall finishes layer by layer: plaster, filler, primer and then paint. Tell us early whether the walls will be painted or papered, because the number of finishing coats depends on exactly that.