How to apply gap filler around skirting boards, windows and doors

You finish laying the skirting, you stand back to have a look, and there's a thin dark line running along the top where the board meets the wall, and from then on it's the only thing you can see. A door frame that's been in the house long enough for the timber to move does the same thing, and so does a window architrave that's pulled a millimetre away from the plaster. Gap filler is what closes those lines up, and it's one of the easier jobs in a house to do well, once somebody has shown you the couple of things that actually matter.

Why the gap is worth filling

The obvious reason is that it looks unfinished, and a filled gap is the difference between a room that reads as done and one that reads as nearly done. The less obvious reason is that a gap is a path, so cold air comes through it in winter and small insects use it, which means you're sealing as much as you're tidying. And if you're painting afterwards, filling first is what lets the paint line run straight instead of dipping into a shadow.

Which filler you actually want

For skirting, architraves, window reveals and door frames inside the house, you want an acrylic gap filler in a cartridge, because it goes on through a caulking gun, it stays slightly flexible so it doesn't crack when the timber moves with the seasons, and you can paint straight over it. That's the one doing most of the work in a house.

A builders filler or a fast filler in a tub is a different product and it's for holes rather than lines, so nail holes, a chip out of a corner, a gouge in the plaster. You press it in with a blade and sand it back flat, and it sets hard instead of staying flexible, which is exactly what you want in a hole and exactly what you don't want in a long joint.

Silicone is the wet area answer, so around a bath or a laundry trough, and the thing to know before you reach for it is that you can't paint over it. If that gap is going to be painted, silicone is the wrong pick no matter how nicely it goes on, and people find that out after the fact more often than you'd think.

What you need in front of you

• Acrylic gap filler in a cartridge
• A caulking gun
• A damp cloth and a bucket of water
• Masking tape, if you want a crisp edge
• A drop sheet, because it does get on the floor
• A smoothing tool, or a wet finger, which works fine

Step 1. Clean the gap out before anything else

Run a dry brush or the vacuum nozzle along the gap and get the dust and the loose paint out of it, then wipe it with a damp cloth and let it dry. Filler sticks to the wall and the timber, and it won't stick to a layer of dust sitting between the two, so nearly every job that lifts off later failed at this step rather than at the clever ones further down.

Step 2. Cut the nozzle small

Cut the tip at about 45 degrees and cut it smaller than you think you need, because you can always take more off and you can't put it back. A small hole gives you a thin bead and a thin bead is easier to control, and if the gap turns out to be wider than that, you cut a bit more off and go again.

Step 3. Tape it if you want the crisp edge

Masking tape on both sides of the gap gives you two dead straight lines and takes about a minute a wall to put on. It isn't compulsory and plenty of people feather the edge by hand and touch it up with paint afterwards, so pick whichever one suits how steady you're feeling.

Step 4. Push it in, don't lay it on

Hold the gun at an angle so the nozzle is pointing into the gap rather than sitting on top of it, squeeze steadily, and move at a slow even pace so the bead keeps up with you. The filler has to fill the void behind the line, and a bead sitting on the surface looks fine for about a week and then shrinks back and shows the gap again.

Step 5. Smooth it while it's still wet

Wet your finger or your smoothing tool, run it along the bead in one pass with light pressure, and push the filler into the joint as you go. Wipe the excess onto the damp cloth straight away rather than letting it build up, because the moment it starts to skin over you're scraping instead of smoothing.

Step 6. Pull the tape wet, then leave it alone

Peel the tape off while the filler is still wet and pull it back on itself at a sharp angle, which gives you the cleanest line. Then leave it. Curing times vary a lot between products and the tube tells you what yours needs, and a cold or damp room stretches that out, so read it rather than guessing and don't paint over it early.

Where it goes wrong

• It cracks along the middle as it dries. The bead was too thick. Two thin passes with the first one dry beats one fat one, every time.
• It won't stick, or it lifts at one edge. Dust, flaking paint or a damp surface. Back to step 1.
• It looks like a dog's breakfast. Practise on a scrap of timber for a couple of minutes first. It's a hand skill and it comes quickly.
• The paint won't take. Somebody used silicone where acrylic belonged, and the only fix is to cut it out and start again.

None of that is hard. It's just unfamiliar, and those are two different problems, so if you've never run a bead before then the honest answer is that your first metre will look ordinary and your fourth one will look like a professional did it.

Shop related products

• GSA Acrylic Gap Filler 450g. The cartridge that does skirting and door frames, and it paints over.
• Uni-Pro 5 in 1 Caulk Tool. Different profiles on one tool for smoothing the bead and cleaning the old stuff out.
• 3 Piece Caulk Finishing Set. For anyone who would rather not use a finger.
• GSA Fast Filler 180g. The tub product for nail holes and chips, sands back flat.
• Soudal All Purpose Silicone 300ml. For the wet areas, where you are not painting over it.

Did you know we also stock caulking guns in store, including a no drip trade gun and a sausage gun, along with Uni-Pro masking tape in 24mm, 36mm and 48mm, the long life blue and the 30 day green tapes, canvas drop sheets, sanding blocks and a silicone remover for when the old bead has to come out first. Those aren't on the website, so give us a ring on (08) 8536 4333 to check what's on the shelf.

Where to go next

If the gap you're filling is part of a paint job, the paint side of it is worth a read too, and we mix and tint Haymes and Taubmans to order in the shop. Have a look at the paint mixing service.

Two others on the same shelf of jobs:

• Soudal T-Rex, what it sticks and when to use it
• Choosing the right adhesive for your DIY project

I still tape both sides even on a short run, because I don't trust my own hand and I'd rather spend the extra minute than spend 10 wiping filler off the wall.

Cheers,
Candeece