Why Butt Joints Are Difficult
Factory drywall edges have a slight taper, a shallow depression that runs the length of the sheet. When two tapered edges meet, they create a trough. You fill this trough with tape and mud, and the result is a surface that's flush with the rest of the drywall.
Butt joints don't have this taper. When two full-thickness edges meet, you're adding tape and mud on top of a surface that's already at full height. The result is a raised area, the crown, that's higher than the surrounding wall.
You can't sand down a crown without going through the paper. The only solution is building out the surrounding area to blend into the crown. This means wide feathering, multiple coats, and careful technique.
Minimizing Butt Joints
The best solution is having fewer butt joints. Plan your layout to reduce them.
Use longer sheets. 4x12 sheets mean fewer end-to-end joints than 4x8 sheets. Yes, they're heavier and harder to handle, but the reduced finishing work is worth it.
Plan layout to put butt joints in less visible locations. High on walls where ceilings cast shadows. In corners where furniture will block the view. Not in the center of a large wall where light rakes across.
Run ceiling sheets perpendicular to joists, which naturally creates fewer butt joints.
Pre-Filling
Before taping, fill the butt joint gap to create a level starting point.
Apply a thin layer of setting compound into the joint. Not over the surface, just in the gap between sheets. Scrape it flush with the drywall surface. Let it set.
This accomplishes two things: it fills any gap so tape lies flat, and it eliminates the void that tape would otherwise bridge unsupported.
For joints with any significant gap, pre-filling is essential. For tight joints with minimal gap, you can skip straight to taping.
Taping Butt Joints
Tape butt joints like any other seam: bed coat, embed tape, let dry. The difference comes in the finish coats.
First coat: Apply mud and embed the tape. The tape creates the initial crown. Don't try to solve everything in this coat. Just get the tape bonded.
Second coat: Here's where you start managing the crown. Apply a wider coat, maybe 8-10 inches total width, with the highest point at the crown. Feather the edges carefully. Let dry.
Third coat: Go wider still, maybe 12-14 inches. The goal is creating such a gradual transition from the crown to the surrounding wall that the eye can't detect it. Feather edges almost to nothing.
Fourth coat if needed: For really stubborn butt joints, a fourth coat feathered to 18+ inches makes the crown essentially invisible.
Wide Knife Technique
Butt joints demand wide knives. A 6-inch knife can't create the gradual feathering needed.
For second coat, use at least a 10-inch knife. For final coats, 12 or 14-inch is better.
The technique: load mud onto the knife and make one pass across the joint, centering on the crown. Then, with almost no mud on the knife, feather out each edge. You're essentially using the knife edge to thin the mud to nothing at the transition.
Multiple light passes beat one heavy pass. Build up gradually rather than trying to apply everything at once.
Checking Your Work
Butt joints are masters of deception. They look fine in normal lighting and terrible when light rakes across them.
Check with a work light at a low angle. Position the light so it shines almost parallel to the wall surface. Every crown and imperfection shows as a shadow.
Run your hand across the joint. You can feel a crown even when you can't see it.
Check from different angles. A joint that looks good from straight on might show from the side.
After each coat dries, check before adding the next. It's much easier to add another feather coat than to try to sand down a finished surface.
Specialized Tools
Butt joint tools exist to help manage the crown.
Butt joint compound: Specialty mud formulated to shrink less, reducing crown height. Works but is expensive.
Butt joint flat boxes: Pro tools that apply mud in a curved profile, automatically creating feathered edges. Expensive but game-changing for high volume work.
Tapered shim strips: Stick to the back of drywall before hanging to create artificial tapered edges. Adds a step but makes butt joints finish like factory edges.
For DIYers, these tools are usually overkill. Good technique with standard tools handles most situations. But if you're doing a lot of butt joints and they're driving you crazy, the specialized tools might be worth investigating.