Filling plasterboard joints and screw heads before painting

What you need before you start

You need joint filler (either ready-mixed or powder you mix yourself), paper or self-adhesive mesh jointing tape, a couple of filling knives of different widths, a sanding block or pole sander with fine-grit mesh or paper, and a work light you can move around.

For most rooms a 100–150 mm knife does the joints and a 250 mm knife does the wide final pass. Ready-mixed filler is easier for a first attempt because it does not set as fast as powder, which gives you more time to work it smooth before it goes off.

Check the screw heads are seated just below the paper surface, not proud of it and not torn through. Any screw sitting high will telegraph through paint no matter how much filler goes over it, so drive it in properly before you fill.

Fitting the joint tape

Run a thin layer of filler into the joint first with a narrow knife, then bed paper tape into it while it is still wet, pressing from the centre outwards to push out air and excess filler. Self-adhesive mesh tape can go straight onto a dry joint without this first layer, which is quicker but not always as strong on inside corners.

Keep the tape straight and centred over the joint. A crooked run is hard to disguise later because every coat after it has to follow the same line to stay flat.

Let this first layer dry fully before building up further coats. On paper tape, wipe off obvious excess with the knife at a shallow angle so you are not left with a ridge to sand down later.

Three coats, each one wider than the last

The first coat, over the tape, is done with a narrow knife and stays close to the joint width. Its job is to lock the tape in place and fill the recess flush.

The second coat uses a wider knife, typically 150–200 mm, and feathers the filler out beyond the edges of the first coat so there is no hard line where it stops. The same applies over screw heads: a slightly wider dab each time, not one thick blob.

The third coat uses the widest knife, around 250 mm, and spreads the filler thinly over an even wider band again. By this point you should barely see a ridge, and running your hand across the joint should feel close to flat before any sanding starts.

Drying and sanding between coats

Each coat needs to dry fully before the next goes on, otherwise the filler underneath stays soft and shrinks later, leaving a dip along the joint. Ready-mixed filler typically wants a day per coat in a normally heated room; powder filler can be workable within a couple of hours but check the setting time on the bag.

Sand lightly between coats with fine-grit paper or mesh on a block, just enough to knock off high ridges and dust, not to cut back into the tape. A dust mask is worth wearing, plasterboard filler dust is fine and gets everywhere.

Wipe or vacuum the dust off before applying the next coat. Filler applied over a dusty surface does not bond as well and can lift later once painted.

Checking the surface with raking light

Once the final coat is dry and sanded, take a work light or a torch and hold it close to the wall at a low, raking angle rather than lighting the wall from the front. Any hollow, ridge, or missed screw head throws a shadow that is invisible under normal room light but obvious under this test.

Mark any shadowed spots lightly in pencil, apply a thin skim of filler there, let it dry, and sand again. It is far easier to fix this before painting than after, since paint sheen makes flaws more visible, not less.

Check from more than one angle and more than one light position, since a single raking light from one side can miss a dip running parallel to the beam.

• Hold the light close to the wall, not from across the room
• Sweep it slowly along the joint line, not just once
• Check horizontally and vertically, especially at corners
• Mark flaws in pencil before you lose the shadow
• Re-check after the final sanding, not just after filling

When to stop and call in a plasterboarder or decorator

Straight joints on a flat wall are a reasonable first DIY job. Inside corners, external corner beads, and ceiling joints are harder to keep flat and are where most amateur jobs show under raking light, because any small error is magnified over a long, visible line.

If the board itself was fitted with uneven gaps, bowed studs, or badly countersunk screws, no amount of filling technique will fully correct that; the underlying fixing needs sorting first. That is a job for whoever hung the board, not something to fill your way out of.

Our plasterboard work starts from 28 € per job for finishing and repairs of this kind, and if you would rather hand over the painting once the walls are flat, our painting and decorating work also starts from 12 € per job. Either is worth considering if the raking-light test keeps turning up problems you cannot get flat.