Acrylic joint sealing

In practice this job comes in two forms that differ slightly.

Sealing joints on ordinary walls and ceilings is mainly part of renovation and redecoration. It covers gaps around door frames, window frames and sills, cracks in the corner between wall and ceiling, and transitions between timber, concrete and plaster. The surface is usually older, often dusty or covered in flaking paint, so cleaning and preparation take more time.

Sealing corners and joints on plasterboard follows the installation of stud partitions, wall linings and suspended ceilings. The flat joints between boards are first filled with jointing compound and reinforcing tape, then sanded. Acrylic comes only after that, into internal corners and wherever plasterboard meets another material, such as the ceiling, masonry or a door frame. The surface is new and flat, so the work goes faster, but a new loft conversion or a large suspended ceiling can mean a long total length of joints.

Why acrylic? Every building moves slightly with changes in temperature and humidity, and new buildings also settle. Hard jointing compound in a corner will crack, while flexible acrylic can take this movement. Unlike silicone, it can be painted over.

The decorator first checks the surface. The joint must be dry, sound and free of dust. Loose bits of plaster and old sealant are removed, old cracks are cleaned out and, if needed, opened up slightly so the sealant has somewhere to flow into. Absorbent or crumbly surfaces are primed before sealing.

The cartridge then goes into a sealant gun. The nozzle is cut at an angle to roughly the width of the gap, and the sealant is applied in one smooth run with steady pressure. It is tooled straight away with a wet finger, a narrow flexible spatula or a smoothing tool, and any excess is wiped off with a damp sponge. After smoothing, the corner should show a thin, straight, barely visible line, not a thick bead.

Deep gaps are not filled with one thick layer. Acrylic shrinks a little as it dries, so a deep gap is either pre-filled first (with jointing compound on plasterboard, or with foam backer rod for wider gaps around door frames), or the acrylic goes on in two thin layers. The second layer is applied only once the first has dried.

Most sealants can be painted after a few hours, but thicker layers or a damp room can need up to a day. The manufacturer's instructions on the cartridge are what count.

• Length and number of joints: a room with a suspended ceiling and several corners means far more running length than a simple box room.
• Width and depth of gaps: narrow gaps are done in one pass, wide ones need pre-filling or several layers.
• Condition of the surface: new plasterboard is ready almost immediately, old cracks have to be opened, cleaned and primed.
• Height and access: corners under high ceilings, stairwells and sloping loft ceilings need ladders or scaffolding and slow the work down.
• Complexity: alcoves, arches, cables and pipes mean many short, awkward sections.
• Finish expectations: with raking light from large windows every imperfection shows, so the work has to be slower and more precise.

• Sealant on dust or damp: it does not bond, pulls away from the edge after a while, and a dark hairline appears in the corner.
• Acrylic instead of jointing compound: flat joints between boards in the same plane need jointing compound and tape. Acrylic does not reinforce them and will show through the paint.
• Too thick a layer: it shrinks, cracks and leaves a visible bead in the corner.
• Smoothing too late: once the sealant has skinned over, it cannot be tooled neatly. Dried acrylic is also hard to sand because it is soft and flexible.
• Silicone instead of acrylic: paint does not stick to silicone and simply beads up.
• Painting over fresh sealant: the paint over the joint can crack or leave patches.
• Skipping the critical spots: the corner at the ceiling, the edge of a suspended ceiling and the area around door frames are the first to crack.

Acrylic has its limits. Larger movement joints, floor joints, outdoor work and areas hit directly by water (a shower enclosure, for example) need a different sealant, usually sanitary silicone or polyurethane. If a crack keeps opening up after sealing or gets wider, it is not a cosmetic problem. Have the cause checked by a professional, and if needed by a structural engineer.

A handy homeowner can do this job. For corners at ceiling height, use a stable stepladder, and for ceilings above normal height it is wiser to call a tradesperson with proper access equipment. Practise the technique on a short, less visible section first.

1. Buy a paintable interior acrylic sealant, and for kitchens and bathrooms (away from direct water) one with higher moisture resistance.
2. Clean the joint, vacuum the dust and remove any loose old sealant and paint.
3. Prime absorbent or crumbly surfaces and let the primer dry.
4. On uneven surfaces or if this is your first time, mask the edges with masking tape.
5. Cut the nozzle at an angle to the width of the gap and load the cartridge into the gun.
6. Apply the sealant in one smooth run with steady pressure, working in sections of about an arm's length.
7. Smooth it immediately with a wet finger or smoothing tool and wipe off the excess with a damp sponge.
8. Pull off the tape while the sealant is still soft.
9. Let it dry as the manufacturer specifies, and top up deeper spots with a second thin layer if needed.
10. Only then start painting.

Look at the joints in raking light, for example with a torch held flat against the wall, before any painting starts.

• Lines in the corners are straight, thin and free of beads
• The sealant holds along its full length and does not peel at the edges
• There are no bubbles, holes or finger marks on the surface
• There are no dried smears of sealant around the joints
• Corners at the ceiling, around door and window frames and at suspended ceiling edges are all sealed
• The sealant is fully dry before painting
• After painting, the paint over the joints covers evenly and does not crack