Joint compound is used in passes, not coats. There is a bedding coat that presses the tape into the joint, a fill coat that brings it level, a second fill because the first one shrinks as it dries, and then whatever it takes to feather the whole thing out far enough that the eye cannot find the joint. Each pass covers the same area with roughly the same weight of compound, so the total is the area times the rate times the number of passes — and the number of passes is the part that separates a joint you cannot see from a joint you can.
Surface, passes and tub
Every pass over the board is another pass of compound.
Walls and ceilings. Joints, screw spots and corners all come out of this.
Taping, fill, second fill. A full skim counts as one heavy pass.
Compound left on the hawk, in the tray, and sanded off the wall.
Result
A joint is not filled, it is feathered
The mistake is thinking the joint is the gap between two boards. It is not. The gap is two or three millimetres wide, and the compound that goes into it is almost nothing. What takes the material is the feather: the compound is spread out several hundred millimetres either side of the joint so that the wall is flat over a wide enough area that the eye reads it as flat. A joint filled flush but not feathered is a visible ridge under raking light.
That is why the rate is quoted per pass over the whole area rather than per metre of joint. On a boarded wall the joints, the internal corners, the external angles and every screw spot all get feathered out into the surrounding surface, and the compound ends up covering a large share of the wall whether or not it is being skimmed.
Why the second fill is not optional
Ready-mixed compound dries by evaporation, and as it dries it shrinks. The fill coat that looked proud when it went on is often slightly hollow once it has dried, and that hollow is exactly the line you can see after painting. The second fill exists to bring the joint back up after the first one has shrunk.
Setting-type compound — a powder mixed with water that sets chemically rather than drying — shrinks far less, which is why it is used for the bedding and the first fill on a big job. It has a working life measured in minutes rather than hours, and it cannot be re-wetted, so it is less forgiving to use; but it needs fewer passes, and on a large area it is quicker overall.
A joint feathered over 150 mm shows. The same joint feathered over 400 mm does not. The compound is the same; the width is the difference.
Use a wider knife for each pass: a 150 mm for bedding, a 250 or 300 mm for the fill, and the widest you can handle for the feather.
Choosing the compound
Ready-mixed general purpose compound is the default for filling and finishing. Taping compound is heavier and sticks better, and it is worth using under the tape. Topping compound is lighter, sands more easily and is the one for the final pass where the sanding is the work. Setting compound is for deep fills, for the bedding coat, and for anywhere the job cannot wait for a drying pass.
There is also a difference between filling a joint and skimming a wall. A skim is one heavy pass over the entire surface at three times the rate, and it is a different job with a different rate, a different tool and a different skill. If the wall needs a skim, do not estimate it as a jointing job.
| Pass | Compound | Rate | Knife |
|---|---|---|---|
| Bed the tape | Setting or taping | 0.18 kg/m² | 150 mm |
| Fill | Setting or general | 0.18 kg/m² | 250 mm |
| Second fill | General purpose | 0.18 kg/m² | 300 mm |
| Feather and top | Topping, light | 0.10 kg/m² | 350 mm plus |
| Screw spots | General purpose | 0.05 kg/m² | 100 mm |
| Full skim | Finishing | 0.30 kg/m² | 600 mm trowel |
A worked example
- One pass50 m² at 0.18 kg/m² is 9.0 kg of compound.
- Three passesTaping, fill and second fill is 27.0 kg.
- AllowanceFifteen percent for the hawk, the tray and the sander takes it to 31.1 kg.
- TubsIn 12 kg tubs that is 3 tubs, or $66 at $22.
- The rate31.1 kg over 50 m² is 0.62 kg/m² — about a third more than the pass rate suggests.
The gap between 0.54 and 0.62 is the fifteen percent, and it is not a guess: a hawk holds compound that never goes on the wall, a tray does the same, and a meaningful share of what does go on comes straight back off on the sanding paper.
Working it
Fill the joint before you tape it, press the tape into the wet compound with the knife rather than laying it on top, and pull the knife along at a shallow angle so the compound is squeezed out from under the tape rather than piled over it. A tape with compound underneath and none on top is a tape that will not bubble.
Let each pass dry fully before the next. Ready-mixed compound that looks dry on the surface is often still wet underneath, and coating over it traps the moisture, which comes out later as a soft patch or a stain through the paint. In a cold room, allow a full day.
Sand between passes rather than at the end, and use a light on the wall at a shallow angle to find the ridges — a sander used in normal room lighting misses most of them. Then the wall area figure gives the surface for the primer, and the paint figure turns it into cans. The boards underneath were sized by the drywall sheet figure, which is also where the joint length comes from.
Do
- Feather each joint out several hundred millimetres, not just over the gap.
- Press the tape into wet compound rather than laying it on top.
- Let every pass dry fully before the next one.
- Sand with a light held at a shallow angle to the wall.
- Use a setting compound for the bedding coat on a big job.
Avoid
- Filling the joint flush but leaving a ridge the light will find.
- Coating over compound that is dry on top and wet underneath.
- Skipping the second fill and painting over the shrinkage.
- Sanding in normal room lighting, which hides every ridge.
- Estimating a full skim as if it were a jointing job.
Where the numbers stop being true
The rate per pass assumes a boarded surface with the usual joint density. A wall of large sheets has fewer joints and uses less; a wall of small sheets, or one with a lot of short pieces around openings, uses more. And a wall with many internal corners uses more again, because a corner is feathered on both faces.
Nothing here accounts for the difference between paper tape and mesh tape, for external corner bead, which is its own material, or for a surface that is being skimmed rather than jointed. Each of those changes the rate rather than the method.
And the waste percentage covers what never reaches the wall, not what gets sanded off. On a job where the finish is being taken back hard to get it flat, the sanding share is much higher than fifteen percent, and the honest figure is nearer twenty-five.
How the figures are produced
One pass is the area times the rate for the chosen job. The pass count multiplies that, and the first pass is reported separately because it is the one that beds the tape. The waste percentage is added, tubs are the total divided by the tub size and rounded up, and the rate output is the total bought divided by the area. The rows draw one line per pass, which is the honest picture of where the material goes.
Questions people ask
How much compound for 50 square metres?
About 31 kg for three passes with a fifteen percent allowance — three 12 kg tubs.
Why three passes?
Because the fill shrinks as it dries. The second fill is what makes the joint invisible after painting.
Ready-mixed or setting powder?
Ready-mixed for small jobs and finishing. Setting powder for bedding and deep fills, and anywhere you cannot wait for a drying pass.
Can I sand between coats?
Yes, and you should. Sand lightly between passes rather than heavily at the end.
How wide should I feather?
Three hundred to four hundred millimetres. Under 150 mm shows under raking light.
Do screw spots count?
Yes, usually three passes each. On a boarded wall they are a meaningful share of the total.