Why Preparation Makes or Breaks an Emulsion Finish

Emulsion is a forgiving paint in many ways. It is easy to apply, dries quickly and cleans up with nothing more than warm water. What it is not, however, is concealing. Because it sits on the wall as a comparatively thin film, it reveals every ridge, hollow and hairline crack lying beneath it. Once low-angle afternoon light rakes across the surface, blemishes you barely registered before painting will suddenly look glaringly obvious.

This is why experienced decorators spend far longer preparing a room than actually painting it. A thorough routine of filling, sanding and priming gives you a surface that accepts paint evenly and looks smooth from every angle, whether the light is flat or harsh. None of it is technically difficult — it simply rewards patience and a bit of care.

Gather the Right Kit Before You Start

Having everything to hand means you are not stopping halfway through a repair to hunt for the right abrasive. A basic preparation kit for a typical room includes:

• Fillers: a fine surface filler for hairline cracks, and a stronger powder filler for deeper holes and damaged plaster.
• Filling knives: a wide flexible blade for skimming flat, plus a smaller one for corners and awkward edges.
• Abrasives: 120 grit for knocking back rough filler, 180 to 240 grit for finishing, wrapped around a cork or foam block to keep things flat.
• Primer: a suitable wall primer, plus a stain-blocker if you are covering old water marks or similar.
• Cleaning supplies: sugar soap, warm water, sponges and lint-free cloths.
• Protection: dust sheets, masking tape, and a dust mask — fine sanding dust travels further than you would expect.

Filling Cracks and Holes the Right Way

Start by understanding what you are filling. A hairline crack caused by gentle settlement behaves quite differently from a deep gouge left behind by a removed shelf, and treating them identically usually means the repair reappears within a season or two.

For hairline cracks, open them up slightly with a filling knife or scraper so the filler can actually reach inside, then brush out the loose dust. Press fine filler firmly into the gap and scrape the surface flush, leaving it just a touch proud — filler shrinks as it cures, and a repair scraped dead flat will end up slightly sunken.

Deeper holes are best built up in thin layers rather than one thick dollop. Let each layer dry thoroughly before adding the next, and only sand back once the final layer has set hard. Rushing this stage is the single most common reason a hollow repair shows through the finished emulsion.

Where walls meet skirting boards, architraves or ceiling lines, a flexible decorator's caulk is generally a wiser choice than filler. It moves with the building instead of cracking as the seasons shift. Run a thin bead, smooth it with a damp finger, and then leave it undisturbed until fully cured.

Sanding for a Genuinely Flat Surface

Sanding is not about scrubbing away at the wall; it is about blending repaired areas invisibly into their surroundings. Work in a circular motion over the filler, then feather the edges outwards so there is no detectable ridge where the repair ends.

Use a block rather than your bare fingers. Fingertips apply uneven pressure and carve hollows into the filler that will show the moment paint hits them. Wipe the wall down with a damp cloth once you have finished, then let it dry completely before moving on — leftover dust will mix with primer and create a gritty, patchy finish.

Check your work by running a bare hand across the surface with your eyes closed. Your palm will find imperfections that your eyes miss. A handheld torch held close to the wall also works wonders, throwing raking light across the plaster and exposing any remaining dips.

Priming: The Step That Prevents Flashing

Bare plaster, filler and old paint all absorb emulsion at different rates. If you skip primer, the repaired patches will drink in the paint faster than the surrounding wall and dry to a duller, darker tone. This unevenness is called flashing, and correcting it means extra coats over the whole wall.

New or bare plaster should be sealed first, typically with a thinned mist coat that soaks in and locks the surface. Filler, being especially porous, benefits from being spot-primed even when the rest of the wall is already painted. Where you are covering stubborn marks, a dedicated stain-blocking primer is essential — ordinary emulsion will let the mark bleed back through, sometimes within days.

Previously glossed or silk-finished walls should be lightly keyed with fine abrasive and given an adhesion primer so the new emulsion has something to grip.

Final Checks Before the Topcoat Goes On

Before you crack open the emulsion, take a slow walk around the room in daylight and then again under a lamp. Look for missed cracks, proud filler edges and any dust you have left behind. If something catches your eye now, it will catch your eye forever once painted.

Touch in any remaining flaws, allow the primer to dry for the full time stated on the tin, and make sure the room is adequately ventilated without being cold. Emulsion applied to a chilly wall can fail to cure properly. Get these unglamorous steps right and the final coat will look effortlessly smooth — which, of course, is the whole point.