Drywall finishing—or taping and mudding—is by far the most challenging part of drywall installation, and it's where amateur drywallers run into the most problems. But while the skill of perfect finishing comes only through practice, there are several common mistakes amateurs make that are easily corrected with a better technique or material, or both.
Problem: Inside Corners Look Ragged
Solution: Try metal-reinforced corner tape.
It combines a metal angle with paper flanges for a perfectly shaped and smooth inside corner. The metal also gives your knife a smooth, rigid surface to ride along when taping the corner.
Problem: Outside Corners Are Rough and/or Fragile
Solution: Instead of using standard paper tape on outside corners, spend a few extra pennies for metal or plastic outside corner bead. Corner bead is more durable and much easier to install than paper-only corners. Save the paper tape for outside corners that never receive any traffic, such as bulkheads or skylight shafts.
Problem: Mud Drying on Inside Corners Before Tape Goes On
Solution: Before you lay the mud, cut your paper tape to length and pre-crease it. You'll save yourself a crucial few seconds. If you're using metal-reinforced tape, cut it to length and have it at the ready before mudding the corner. In either case, it's also easier to measure your tape because there's no mud on the wall yet.
Problem: Drywall Tape Shows Through the Mud
Solution: Remember, it's three coats of mud (at least): tape coat, filler coat, and final coat. It might be that you are excluding the final coat(s). The tape actually should show through the filler coat; if it doesn't, your filler coat is too thick. Covering the tape comes only with the final coat(s).
If your first final coat doesn't cover the tape, apply more coats, but keep them thin.
Problem: Can't Hide Mesh Tape on Butt Joints
Solution: This can be hard to fix and is why a lot of drywallers prefer paper tape on butt joints. To be clear, butt joints are seams where two panels meet at their square (not tapered) edges, so there's no recess for the tape and mud. If you've already used mesh tape and mudded over it (and it's still showing), keep applying thin final coats with a 10- or 12-inch drywall knife, gradually building up the joint in the middle—to cover the tape—and feathering it out at both sides to make it all look flat.
Problem: Can't Hide Paper Tape on Butt Joints
Solution: Butt joints are difficult, even with paper tape. Since you have no edge tapers creating a recess for the tape and mud, the tape is riding on top of the drywall. Yes, it takes a patient hand, but you do need to carefully apply the final coat(s) and feather it out well with your 10- or 12-inch knife. The biggest mistake with butt joints is to use too much mud under the tape, creating a hump that is hard to hide with all the thin final coats.
Problem: Drywall Seams Cracking
Solution: Drywall joints crack for different reasons.
Over door and window openings, joints often crack due to movement in the wall framing. The movement is unavoidable, so the best solution is to make the taped joints as strong as possible. Paper tape itself is less strong than mesh tape, but paper is also less elastic and doesn't stretch like mesh tape does. This is why some drywallers prefer paper for these high-stress joints. If you use mesh tape in these areas, always use a setting-type mud for the first coat over the tape. Setting mud is stronger than pre-mixed all-purpose mud and helps compensate for the elasticity of mesh tape.
Any drywall joint can also crack if the mud is applied too thick or too quickly. To prevent cracking, don't use more mud than you need for any of the coats, and let each coat dry completely before adding the next.