How tape and jointing works
Three coats, each wider than the last, each sanded lightly. The width is what hides the joint, not the thickness.
Three coats, each wider than the last
The first coat beds the tape into compound over the joint, using a narrow knife. The second coat, once dry, goes on wider with a bigger knife and covers the tape. The third is wider again, thin, and is what feathers the edge out to nothing.
The instinct is to build thickness and then sand it flat, and that is the wrong approach entirely. What hides a joint is width, because a gentle taper over 400mm is invisible while a thick ridge over 100mm shows under any light.
- Coat one, 150mm knife
- Bed the tape. Enough compound under it to fill, and squeeze the excess out from under the tape.
- Coat two, 250mm knife
- Cover the tape completely and start the taper. Once the first coat is fully dry, not before.
- Coat three, 300mm or wider
- Thin, wide, feathered to nothing at the edges. This is the coat that makes it invisible.
- Width, not thickness
- A taper over 400mm cannot be seen. A ridge over 100mm shows under every light in the room.
Paper tape or mesh
Paper tape is stronger and it is what a flat joint wants, because it has no stretch and it resists the joint cracking as the building moves. It must be bedded into wet compound, which is the step people find awkward.
Self adhesive mesh is easier because it sticks straight to the board, and it is weaker and slightly thicker. It suits patches and small repairs. For internal corners, paper tape with a crease down the middle is the only sensible option.
- Paper for flat joints
- No stretch, so it resists cracking. Bedded into wet compound rather than stuck to dry board.
- Mesh for patches
- Self adhesive and easier, weaker and thicker. Fine for repairs, not for a whole room.
- Paper for internal corners
- Creased down the centre. Mesh cannot form a crisp internal angle at all.
- Metal or paper faced bead for external corners
- An external corner takes knocks, and tape alone will not survive them.
Setting compound and air drying compound
Air drying compound, sold ready mixed, dries by evaporation. It sands beautifully and it is slow, sometimes overnight between coats in a cold room. Setting compound is mixed from powder and chemically sets in a stated time, so several coats can go on in a day.
The usual approach is setting compound for the bedding coat where strength matters and speed helps, then ready mixed for the finishing coats where sanding easily matters more. Never mix new material into a batch that has started to go off.
- Ready mixed for finishing
- Sands easily and gives the best final surface. Slow to dry, especially in a cold room.
- Setting compound for bedding
- Mixed from powder, sets chemically in a stated time, so a room can be coated in a day.
- Never retemper setting compound
- Adding water to a batch that has begun to set gives a weak coat that will crack.
- Clean tools between coats
- A dried lump on a knife edge draws a line through every subsequent coat you apply.
Sanding, and doing less of it
Good jointing needs very little sanding, and heavy sanding is a sign the coats were applied too thick. Sanding through into the paper face of the board raises a fuzz that shows through paint permanently, and it cannot be fixed by sanding more.
Use a fine mesh or 120 to 150 grit on a pole sander, work with a raking light across the wall so the shadows show what is left, and stop when it feels flat under your hand rather than when it looks flat.
- Less sanding means better jointing
- If you are sanding hard, the coats went on too thick. Fix the application, not the surface.
- Never sand into the paper
- It raises a fuzz that shows through paint and cannot be sanded out, only sealed and refilled.
- Raking light
- A lamp held flat to the wall throws a shadow off anything proud. It is the only reliable check.
- Feel it
- Run a flat hand over the joint. Your hand finds a ridge your eye will not, under any light.
- Mask and extraction
- Joint compound dust is very fine and it travels through a whole house. A vacuum sander is worth hiring.
Corners are where it goes wrong
An internal corner is two tapers meeting at right angles, and the temptation is to work both sides at once. Doing one side, letting it dry and then doing the other gives a much crisper angle, because the knife has a firm edge to run against.
External corners need a bead. A metal or paper faced angle bead sets a straight true line and takes knocks that would destroy tape alone, and it is bedded in and then coated the same three times as a flat joint.
- One side at a time
- Let it dry before doing the other. Two wet sides pull each other out and lose the angle.
- Corner trowel for consistency
- A 90 degree blade does both faces at once on the finishing coat, once the angle is established.
- Bead every external corner
- It gives a straight line and survives being knocked, which tape on its own will not.
- Check the corner is actually square
- Many are not. A bead follows the wall, so the taper is what corrects the eye.
Frequently asked questions
- What tools do I need for tape and jointing?
- A 150mm and a 250mm taping knife, a jointing trowel, a mud pan or hawk, paper tape, a mixing paddle and bucket, a pole sander with fine mesh, and a corner trowel. That set will do a room properly, and everything else on the list makes it faster rather than better.
- How many coats does a plasterboard joint need?
- Three. The first beds the tape, the second covers it and starts the taper, and the third is thin and wide and feathers the edge to nothing. Each coat is wider than the last, because width is what hides a joint rather than thickness.
- What is the difference between paper tape and mesh tape?
- Paper tape has no stretch, so it resists cracking as the building moves, and it must be bedded into wet compound. Self adhesive mesh sticks straight to the board, which is easier, but it is weaker and thicker. Use paper for whole rooms and internal corners, mesh for patches.
- Should I use setting compound or ready mixed?
- Setting compound for the bedding coat, because it is stronger and sets in a stated time so you can carry on the same day. Ready mixed air drying compound for the finishing coats, because it sands far more easily and gives a better final surface.
- Why can I see my joints after painting?
- Almost always because the coats were too narrow, so there is a visible ridge rather than an invisible taper. It can also be sanding through into the paper face, which raises a fuzz, or paint sheen differences from not sealing the compound before painting.
- How do I get a crisp internal corner?
- Do one side, let it dry completely, then do the other. Two wet sides pull each other out of shape. Use creased paper tape rather than mesh, and once the angle is established a 90 degree corner trowel finishes both faces at once.
- How much sanding should tape and jointing need?
- Very little. Heavy sanding means the coats went on too thick, and the fix is thinner wider coats rather than more sanding. Sanding into the board's paper face raises a fuzz that shows through paint and cannot be sanded away.
- Is tape and jointing the same as skimming?
- No. Skimming covers the entire wall with a thin plaster coat. Tape and jointing treats only the joints and screw heads, feathering them out so the board face itself is the finished surface. Jointing is less skilled and less messy, and it needs the board face to be undamaged.
