This page contains instructions for finishing inside drywall corners using paper tape and ready-mixed joint compound.
We use ready-mixed joint compound for this job because setting-type compound will harden too quickly, causing problems if you don't move fast enough to finish before it sets. Ready-mixed comes wet in a bucket and it dries rather than setting through a chemical reaction like setting compound. A small, 1-gallon tub will finish a ceiling or one wall but to do a whole room a 5-gallon bucket will be needed.
Use a roll of paper drywall tape to finish inside corners, not fiberglass mesh. Start by cutting a piece of tape a little longer than each each corner to be finished, fold it along the center crease and set it aside.
Use a 6 inch joint knife and mud pan to apply a thick coat of joint compound to both walls. Fill from the corner out onto the drywall several inches with about ¼ inch of mud from one end to the other.
Starting at one end, place the squared end of the tape into the mud so it's butted to the ceiling or adjacent wall. Unfold the it slightly and push it into the mud along the length of the corner. Press the paper gently into the mud so it makes good contact with both sides. At the end of the corner cut the excess tape to fit squarely against the floor or adjacent wall.
Use the joint knife and working on one wall at a time, stick the tape down tightly. Hold the blade at about 30° and pull it along the corner, pressing down to squeeze the excess mud from between the tape and drywall. Finish one wall and then do the other one. Keep the side of the blade riding against the corner to be sure the mud is pushed out to the outer edge. Scrape the collected mud from the knife blade onto the mud pan frequently to keep it clean and avoid spreading joint compound to unwanted places.
To insure the tape won't come loose over time, go over it again, pressing harder with the blade to squeeze out most of the mud and check that the tape is flat with no bubbles or gaps. If you leave too much mud under the tape it can pop free with the normal expansion and contraction in the wall.
Be careful not to tear the tape with repeated strokes. If the tape bunches up or tears, straighten it out and smooth it again with the knife, add a little more joint compound if necessary. Be sure the tape is tight and smooth against the drywall along the whole corner before moving on.
Wait about 12 hours to allow the mud to dry completely and then apply a second coat. Use the same joint knife to apply a thick coat over the tape, on both walls. Spread the mud to completely cover the tape, the first mud coat and out onto the drywall.
Start at one end, and running the knife blade against the corner, skim the excess mud leaving a thin coat that covers the tape. Do one wall completely and then do the other one. Don't be concerned that the mud doesn't come together at the corner in a perfectly smooth finish. The edge of the knife blade will ruffle the point where the two meet, this will be corrected in the next step.
Let the second coat sit for several hours or overnight, until it's completely dry. Use the joint knife to scrape any dry, rough compound from the corner and then apply a thick coat of mud to one wall. Immediately skim off the excess to smooth out the finish working from one end to the other. Let the mud dry completely.
When it's dry use the joint knife to scrape off any buildup in the corner and then apply a thick coat of mud to other wall. Immediately skim off the excess as before, working from one end to the other. Along the edges go over the mud to remove any buildup there and smooth it flat to the drywall. Let the mud dry.
When it's completely dry lightly sand with 120 grit medium sandpaper to smooth the surface and feather the edges into the surrounding wall. Be careful along the edges not to dig into the drywall paper. Wipe the dust from the walls with a damp cloth before priming and painting. If there are pits and scratches in the finish use a high-build drywall primer, otherwise one coat of regular drywall primer will do if you're using flat latex finish paint. If you are using semi-gloss or another shiny paint, use 2 coats of primer.