Mastic and Caulking Around Skirting Boards

The main reason is appearance. With the top gap filled, the skirting looks like part of the wall, small bumps in the plaster disappear and you get a clean line around the room. There is also a practical reason. An open gap traps dust, grit from shoes and small insects, and a vacuum cleaner struggles to get them out.

As a rule, only the top gap between skirting and wall is filled. With a floating floor (laminate, click vinyl, engineered boards), the gap between the skirting and the floor is left open. A floating floor has to move freely as temperature and humidity change, and caulk would tie it to the skirting. In practice this ends with torn caulk or skirting that lifts away from the wall. With tiles or glued-down floors the bottom gap is sometimes sealed, but then the material depends on whether water can reach that spot.

Acrylic and silicone look much the same in the cartridge, but they do different jobs.

• Acrylic caulk (often sold as decorators' caulk) is only slightly flexible, can be painted, sticks well to plaster, plasterboard, wood and painted plastic, and smooths easily with water. It is meant for dry rooms: living room, bedroom, hallway.
• Sanitary silicone is more flexible and handles water much better, but standard silicone cannot be painted. It belongs where the floor is mopped very often or where water can splash, for example in a bathroom or next to a shower.

So acrylic is not a waterproof seal. It copes with normal mopping, but standing water softens it over time. If your skirting is oiled natural wood or a wood-effect plastic, white caulk will show. In that case choose a caulk in a matching colour, or fill only the places where there is a visible gap.

The simplest case is new skirting against a freshly decorated, straight wall. The gap is thin and even, so one pass and a quick smoothing is enough. The job takes longer when there is:

• an old joint with cracked caulk or flaking paint that has to be cut out and cleaned first
• an uneven wall where the gap is several millimetres wide in places and needs filling in two passes
• a room with lots of corners, door frames and heating pipes
• dark skirting against a light wall, where the bead has to be very neat or run between masking tape
• a request to paint the joints once the caulk has dried

If the gap is wider than one layer can handle, a foam backer rod goes into it first and the caulk is applied over it. Acrylic shrinks a little as it dries, so a thick bead often sinks or cracks.

You can do this yourself. All you need is a caulking gun, a cartridge of acrylic caulk, a utility knife, a damp sponge and some patience. Work at normal room temperature. Most manufacturers print the minimum application temperature on the cartridge.

1. Cut out any old cracked or loose caulk with the knife and remove flaking paint.
2. Vacuum the gap, wipe it with a slightly damp cloth and let it dry.
3. Apply masking tape to the wall and the skirting just either side of the gap, if you want a very sharp line.
4. Cut the nozzle at an angle so the opening matches the width of the gap.
5. Press foam backer rod into any wide gaps first.
6. Run the caulk in one smooth pass over a section of about one to two metres, moving the gun steadily without stopping.
7. Smooth the bead straight away with a wet finger or a plastic caulk tool into a slightly concave shape.
8. Wipe off excess caulk with a damp sponge before it sets, and pull off the tape while the caulk is still wet.
9. Move on to the next section and join neatly onto the previous bead at corners.
10. Let the caulk dry as the manufacturer states, usually a few hours to a day, before painting over it.

The same problems come up again and again:

• Caulking over dust or damp. The caulk only sticks to the dirt and comes away after a few weeks.
• Too much caulk at once. A thick layer dries slowly, cracks and looks like squeezed-out paste rather than a clean transition.
• Not smoothing the bead. Lumps, marks where the gun stopped and a rough surface are left behind, and dirt clings to them.
• Sealing the bottom gap on a floating floor. The floor loses its room to move and the caulk tears.
• Using acrylic in wet areas. Next to a shower or bath it softens, turns black with mould and falls out.
• Painting too soon. Paint on fresh acrylic cracks and leaves patchy marks along the joint.

Look at the finished joint in side light, for example with a torch laid on the floor. This shows up every bump and hollow.

• The gap is filled along its full length with no breaks or holes
• The caulk surface is smooth, even in width and free of lumps
• There are no dried smears of caulk on the skirting or the wall
• Beads join cleanly at corners and door frames
• The caulk is stuck on both sides and does not peel when pressed with a finger
• The bottom gap on a floating floor has been left open
• Silicone, not acrylic, has been used in wet areas
• Painted joints have an even colour and no cracks