When drywall damage exceeds what spackling compound can handle — holes larger than six inches, long cracks, or full sheet replacements — the repair requires cutting in a new piece of drywall, applying paper or mesh tape over the seams, and building up joint compound ("mud") in multiple thin coats. Each coat is feathered wider than the last (typically 6, 10, then 12 inches) to create a gradual transition that disappears once sanded and painted. The quality of the final surface depends almost entirely on patience during the drying and sanding stages.
Practice on a scrap piece of drywall propped against a wall before tackling a real repair. Apply tape and three coats of mud, sanding between each, then hold a work light at a sharp angle against the surface — this "raking light" reveals every ridge and imperfection the way normal lighting never will.
From your wall patching work, you know how to fill small holes with spackling compound. Drywall taping and mudding is that same idea scaled up — applied to the seams between sheets of drywall, corners, or large-patch repairs. The central problem is that drywall is installed in panels, and every edge where two panels meet is a potential crack site. The solution is to bridge that seam with tape embedded in joint compound, then build up the surface around it until the joint becomes invisible. This is less about filling a hole and more about creating a gradual, feathered transition that the eye and hand cannot detect.
The process has three distinct phases: taping, coating, and finishing. Taping first: apply a thin layer of joint compound (called "mud") over the seam, press your paper tape into it, and smooth it flat with a 6-inch knife. The mud acts as adhesive; the tape provides tensile strength across the joint. Once dry, apply a second coat wider than the first — typically 8–10 inches — this time without tape, just mud. The third coat goes wider still, 10–12 inches, applied as thinly as possible. Each coat must be fully dry before the next; joint compound turns from grey-white to pure white when dry, which is your reliable visual indicator. Between coats, sand lightly with 120-grit paper to knock down ridges before adding more mud.
The feathering principle is the skill that makes this work. A seam is slightly raised above the surrounding drywall; your job is to make that bump imperceptible by spreading the mud out over a wide area so the transition happens gradually. If you apply mud only over the seam itself, you'll see a visible hump on the finished wall. By spreading wider with each coat, you're creating a ramp so long and gradual that no single point of transition is visible. This is why your 6-inch knife for the first coat gives way to a 10-inch and then 12-inch knife for subsequent coats — wider knives naturally force you to spread wider.
Finishing begins with a critical quality-check step: raking light. Hold a work light or phone flashlight at a sharp angle to the wall surface, nearly parallel to it. In normal overhead lighting, even bad mudwork can look acceptable. In raking light, every ridge, hollow, and trowel mark casts a shadow and becomes unmistakable. Sand anything the raking light reveals with 150-grit paper, wipe the dust away, and apply a thin skim coat to any low spots. Only when the surface passes the raking-light test is it ready for primer and paint. Skipping this test and discovering the problems only after painting is the most common and expensive mistake in drywall work.