Guide
Drywall mud: which one, and how much
Four kinds of joint compound, the coat each one is for, and the quantities by finish level.
Use a setting-type or taping compound for the first coat, where strength matters, and a lightweight all-purpose or topping compound for the coats you will sand. One 4.5 gallon bucket finishes about 9 sheets of 4 × 8 at Level 4.
The four kinds, and which coat each one is for
| Compound | How it hardens | Best for | Weak point |
|---|---|---|---|
| Setting type (hot mud), powder | Chemical set, sold by working time: 5, 20, 45, 90 minutes | Embedding tape, filling deep gaps, a second coat the same day, bathrooms | Hard to sand; mix only what you can use before it sets |
| Taping compound, premixed | Air dries | Bedding tape; the strongest of the premixed kinds | Heavy to sand, so a poor finish coat |
| All-purpose, premixed | Air dries | Every coat on a small job where one bucket is simpler | Shrinks more than setting type in thick fills |
| Lightweight all-purpose or topping | Air dries | Fill and finish coats, skim coats | Weaker bond, so not for embedding tape |
The combination most finishers settle on is a 45 or 90 minute setting compound for the tape and the first fill, then lightweight all-purpose for the last two coats. The setting compound does not shrink and bonds hardest; the lightweight one sands to a finish in a fraction of the time.
How much mud you need
Compound is set by the finish level, not by the size of the room, which is why two rooms with the same sheet count can need very different amounts:
| Finish level | Gallons per sheet | Sheets per 4.5 gal bucket |
|---|---|---|
| Level 1: fire tape only | 0.19 | 23 |
| Level 2: garage or utility | 0.29 | 15 |
| Level 3: standard paint | 0.38 | 11 |
| Level 4: flat paint, light | 0.48 | 9 |
| Level 5: gloss or critical light | 0.67 | 6 |
The drywall calculator works this out for a room, and the whole-house estimator for every room at once. Tape runs at about 38 linear feet of joint per sheet, so a 500 ft roll covers roughly 13 sheets.
Why drywall mud cracks
- It went on too thick. Premixed compound shrinks as the water leaves it, and a thick fill shrinks enough to crack. Fill deep gaps with setting compound, or in thin passes.
- It dried too fast. A heater or a fan pointed at fresh mud dries the surface before the inside, and the skin cracks as the rest shrinks under it.
- There is no tape under it. Compound alone cannot bridge a joint that moves. A crack along a seam that was mudded without tape will come back however many times it is filled.
- The joint is not on framing. A seam floating between studs flexes, and the mud over it cracks within a heating season.
Why it bubbles
Bubbles in the coat are air: from mixing too fast with a drill paddle, or from mud pressed over bare, dusty paper that pushes air back out as it is absorbed. Mix at low speed, wipe the dust off, and on a skim coat thin the first pass slightly so it wets the paper. Blisters under the tape itself mean the tape was not bedded in enough compound, and those have to be cut out and re-taped.
What it will and will not stick to
Compound bonds well to drywall paper, to itself and to flat or eggshell paint that is clean and sound. Glossy paint needs sanding dull or a bonding primer first. It does not bond reliably to bare wood or plywood: wood moves with humidity and the joint cracks, so fill wood with wood filler instead. Over old plaster, a bonding primer first is the difference between a patch that lasts and one that lifts.