Drywall is pressed gypsum, which is used in modern walls as the surface material. Cement board is also used in the walls of showers and bath surrounds, where there's going to be a lot of moisture. Cement board is much heavier and harder to work with than drywall, but it is almost completely waterproof. If you have covered a bath surround in cement board and it's meeting the rest of the wall in the bathroom, you will at some point have seams between the drywall and the cement board. In the process of "taping" the drywall and the cement board, the goal is to make them look like one surface.
Instructions
1. Apply a length of mesh-style, self-adhesive drywall tape lengthwise over each of the seams that are between the drywall and the cement board, rolling the sticky side of the tape out along its length and pressing it firmly to the surface with your palm. The tape should be positioned so the seam is right along the middle of it, with the width of the tape sitting along the edges of both boards.
2. Cut off the end of the tape to fit with a utility knife.
3. Spread joint compound over the length of the tape with a 6-inch drywall knife. Make the seam just wide enough to cover the entire surface of the tape (generally 3 inches). Get it smooth and flat. Dab additional compound over the screw holes on both the drywall and cement boards, near the seam but not under the tape.
4. Let the compound set for six hours. Use the 6-inch utility knife to lightly scrape off any high points or ridges.
5. Apply a second layer of compound over the lines, this time with a 10-inch utility knife so that the seams are 5 or 6 inches wide. This application should completely cover the previous seam and extend beyond it, along the top and bottom. Get the layer thin and smooth. Let it dry for six hours.
6. Scrape the second layer. Add a third layer, making it about 8 inches wide and very flat. Let it dry overnight. Sand it smooth. The wall is now ready for tile or paint.
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