Always be sure to have the following safety items available for your protection: safety glasses, gloves, and a proper dust mask or respirator.
Wallboard Joint Compound
This can be purchased pre-mixed in a thick slurry. Usually a 5-gallon can holds enough to finish the seams of approximately fifteen 4' x 12' panels if you vary the consistency of the layers. You may want to thin the compound a little before you start taping with it, but don't make it too thin. When it has the right consistency it will stick nicely to your knife and spread easily. As you work with it, you will notice that it will begin to stiffen after a little while, so you may need to stir in more water occasionally. You can experiment with how much water to use, but a good start would be about two or three tablespoons of water to a pan full of joint compound.
Basic Technique for Applying Joint Compound
It takes practice to learn the right consistency for each layer or how to use the broadknife with finesse. The best way to learn how to load the knife with compound, and how to spread and feather it would be to watch someone else do it, either in person or on video. This article will only touch on the basics to give you an idea about what to expect during this process.
Scoop a 5-inch drywall taping broadknife sideways into the compound and load only half the width of the blade. Center the blade over the joint so that the compound is on the leading edge and spread a smooth, thin layer of joint compound along the length of the seam, angling it toward the tape, closer and closer to the plasterboard, as you steadily drag it along the seam, forcing the compound into the depression created by the tapered edges of the plasterboard. The seam should be as flat as you can make it. Use just enough joint compound under the tape to hold it well. Make sure there are no humps, bumps or skips in the compound because if the layer is inconsistent or too thin, the tape will not stay put. Even tiny skips can cause blistering or bubbling problems that can show up later.
After applying first layer of joint compound, let it dry completely following the instructions on the compound that you have purchased. Lightly sand it down with fine sandpaper to get rid of any bumps or ridges before applying the next layer. Don't sand so hard that you dig into the tape or the surrounding drywall. Brush away any loose crumbs and apply the next layer of compound with next larger knife, usually 8-inches.
Feather the second layer of joint compound, extending the edges out to about 6-8 inches beyond the center of the tape or fastener head. Let it dry and sand lightly again. Brush away any loose crumbs and apply the third layer of compound with next larger knife, usually 10- or 12-inchess.
The third coat is wider than the second one and takes the feathered edges out to 12 inches. Let it dry and then sand it. Since this is your last chance to get the sanding correct, be very careful to make it smooth.
Embedding the Tape
Center one end of the tape over one end of the joint and press it into the wet joint compound. If the tape is not centered, your efforts are wasted. While guiding the tape with one hand, run the blade of a 5-inch broadknife along the joint to force the tape against the wet compound. When you reach the other end of the joint, press the knife tightly into both tape and wallboard and use it as a straight-edge for tearing off the excess tape. Repeat this procedure with all the seams. Let dry thoroughly before adding second and third coats of joint compound to the joints.
Taping the Inside Corners
Run the knife along one side of the corner joint, angling the loaded edge of the knife into the corner to create a slightly thicker layer of compound right at the seam. Repeat with the other side and do not worry if you scrape off a bit of compound from the first side.
Fold the tape along its lengthwise pre-creased mid-line and press women's jackets it lightly into the wet compound using your fingers to force the crease into the corner seam from one end to the other along the joint. Glide the 5-inch knife lightly over the surface of the tape, first along one side of the crease and then along the other side. Press just lightly enough to make the tape stick to the compound. Repeat this process using more pressure to squeeze out the excess compound along the seam. Finally, coat the tape lightly with some of the excess compound and glide the knife over one last time on both sides to leave a thin layer of compound on top of the tape.
Hiding Fastener Heads
Hold the blade almost flush against the plasterboard and draw the knife with the compound across a fastener head and the dimple surrounding it. Then scrape off the excess with a stroke that is at right angles (perpendicular) to the first. Repeat on all other fastener heads. Apply second and third coats in the same manner after each coat has thoroughly dried and shrunk a bit. After the 3rd coat, the dimple should not be visible when dry.
Covering a Corner Bead
Load about 2/3 of the width of a 5-inch broadknife with joint compound and lap the blade about two inches over the outside of the corner. Apply the compound by gliding the knife along one face of the bead. Repeat on the other face. Then scrape off the excess compound from your knife and smooth the joint by alternately running the knife down the two faces. Repeat with the second and third layers and be sure to feather the layers.
Feathering a Joint
Load joint compound onto the full width of an 8-inch knife and spread the second layer of compound over the first. Then, holding the knife slightly off-center so that it laps the joint unevenly, slide it down the joint again. This time bear down on the knife edge that is farthest from the joint to feather the compound on that side. Repeat with the other side. Apply the 3rd layer with a 12-inch knife.
Feathering an Inside Corner
Load joint compound onto the full width of a 5-inch knife and spread the second layer to one side of the corner. Scrape off any compound that laps onto the second side then slide the knife down the first side again, bearing down on the knife edge that is farthest from the joint to feather the compound on that side. Repeat with the other side.
Cleaning up
Before resealing a partially used container of joint compound, wipe off the interior sides and smooth the surface of the compound. If you aren't going to be using it for several days, you can pour 1/2 inch of water onto the surface to keep it from drying out in the can, and then just pour it off when you re-open the container.
After all the layers are done on all the joints and fastener heads just let everything dry completely before painting or papering the walls. With normal heat, this is about 8 to 12 hours, depending on humidity, but follow instructions on product container. Keep the room temperature above 50 degrees. A household fan or two set around the job can speed things up...and you are done!
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