Finishing
Joint compound: all-purpose, lightweight, setting, and gallons per sheet
One bucket per ten sheets is the rule everyone quotes and nobody can source. Here is what the makers publish, and how the finish level changes it.
Two families of compound
Joint compound, or mud, comes in two families that harden in different ways.
Ready-mixed (drying-type) compound comes wet in pails and boxes. It hardens as its water evaporates, so it takes a day or so per coat and shrinks a little as it dries. It is easy to work, sands easily, and keeps for months in a sealed pail.
Setting-type compound comes as a powder in bags. Mixed with water, it hardens by a chemical reaction, like plaster, in a set time printed on the bag: 5, 20, 45 or 90 minutes. It shrinks less, hardens whatever the humidity, and can be recoated as soon as it has set. It is harder to sand, especially the standard (non-lightweight) kind.
The products within each family
| Compound | What it is for |
|---|---|
| All-purpose ready-mix | Every coat, including bedding tape; the usual choice for DIY |
| Lightweight all-purpose | Every coat; lighter, sands more easily, shrinks a little more |
| Topping compound | Finish coats only; smoothest, easiest sanding, weak for bedding tape |
| Taping compound | Bedding tape and first coats; strongest bond, harder to sand |
| Lightweight setting (e.g. Easy Sand) | Tape coat, fills, repairs; sandable |
| Standard setting (e.g. Durabond) | Tape coat, deep fills, repairs, mesh tape; very hard, hard to sand |
A common professional approach uses setting compound for the tape coat, which is strong and sets fast, and lightweight ready-mix for the finish coats, which sand easily. Mesh tape must always be bedded in setting compound; the guide to paper vs mesh tape explains why.
How much: the published figure
USG publishes the coverage of its ready-mixed all-purpose compound for complete finishing, tape coat plus two finish coats on the joints and three on the fasteners, which is a Level 4 finish: 9.4 gallons per 1,000 sq ft of board. USG’s own FAQ rounds it to about two 4.5-gallon pails per 1,000 sq ft. The site uses that figure for Level 4 and pro-rates the other levels by the passes each one adds:
| Finish level | Gallons per 1,000 sq ft | Gallons per 4 × 8 sheet |
|---|---|---|
| Level 2 | 4.2 | 0.13 |
| Level 3 | 7.0 | 0.22 |
| Level 4 | 9.4 | 0.30 |
| Level 5 (with skim) | 14.0 | 0.45 |
At Level 4 a 4.5-gallon pail finishes about 15 sheets of 4 × 8; the folk rule of one pail per ten sheets builds in waste and heavy coats. The compound coverage table has the full figures with setting compound and tape.
A worked job
40 sheets of 4 × 8, Level 4
- Board: 40 × 32 = 1,280 sq ft
- Compound: 1,280 × 0.94 ÷ 100 × 1.1 waste = 13.2 gallons, 3 pails of 4.5 gal
- By pass: tape coat 3.4 gal, coat over the tape 2.5 gal, second coat 3.9 gal, finish coat 3.4 gal
- All in setting compound instead: 5 bags, at about 340 sq ft of board per 18 lb or 25 lb bag
- Tape coat in setting compound, the rest in ready-mix: 2 bags and 3 pails
The joint compound calculator gives the same breakdown for any area, level and compound.
Pails, boxes and bags
Ready-mix comes in 4.5-gallon pails, which weigh about 60 lb (lightweight pails less), and in boxes of about 3.5 gallons, which cover proportionally less. For repairs, quarts and gallons. Setting compound comes in 18 lb bags of the lightweight kind and 25 lb bags of the standard kind; USG lists about 340 sq ft of board per bag at full finishing.
Buy what the job needs plus a little: an opened pail keeps for weeks if its lid is sealed and the surface is covered with a film of water, but a bag of setting compound that has absorbed moisture sets unpredictably.
Mixing setting compound
Setting compound is mixed with clean, cool water, in a clean bucket, only as much as can be used before it sets. USG lists about 5.2 quarts of water per 18 lb bag of lightweight 45-minute compound.
Half a bag of Easy Sand 45
- 9 lb of powder × 5.2 qt ÷ 18 lb = 2.6 quarts of water
- About 45 minutes from the first contact of water and powder to set
- For 544 sq ft of board, one bag covers the tape coat, in two batches
The setting compound mixing calculator scales the water for any product, bag and batch.
Water first, powder second, mixed about a minute to a smooth paste. Warm water and a warm room shorten the set; leftover set compound in the bucket shortens it a lot, so clean the bucket between batches. Never add water to compound that has started to stiffen.
Thinning ready-mix
Ready-mix straight from the pail is stiff for the tape coat and right for fills. For finish coats and skim coats, most finishers thin it with a little water, a cup or two per pail, mixed with a paddle until it spreads like thick yoghurt. Too thin, and it shrinks and runs; thin only what you will use. A skim coat over a whole wall takes thinned compound in thin passes, and the skim coat calculator counts it.
Drying, and why it rules the schedule
Ready-mix dries in about 24 hours at 70 °F and 70% humidity, by USG’s figure, faster in dry warm air and much slower in cold, damp rooms. Setting compound hardens in its set time regardless of the air, which is why it is used for tape coats and fills when the schedule is tight. The drying time calculator estimates a coat from your room’s conditions, and the guide to drying times between coats covers the rest.
Older compound and asbestos
Joint compound made before about 1980 may contain asbestos. Before sanding, cutting or demolishing old finished walls, have the compound tested; the US EPA’s guidance explains what to do if it is positive. New compound sold today does not contain asbestos.
Common mistakes
- Ready-mix over mesh tape. It cracks; bed mesh in setting compound.
- Topping compound for the tape coat. It does not hold the tape.
- Mixing setting compound into ready-mix. It sets the whole pail.
- Thick coats. They shrink, crack and take days to dry.
- Buying by the folk rule. Count by area and level.