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In this example, some of the original ceiling had grown brittle beyond repair and released from the lath, so it had to be removed. After taking out the crumbling plaster, remove the cove molding and the lath, if necessary. Replace the missing plaster with drywall cut to size (Image 1). Attach the drywall using drywall screws and a screw gun (Image 2).
Once the wallpaper is removed from the walls, assess the plaster walls (Image 1). Before priming and painting the walls, apply a light skim coat with gypsum compound to seal them. Place a small amount of compound on the application paddle, then scrape the compound off the edges of the blade before applying compound to the wall. In that way, excess compound doesn't "bleed" over the edges of the blade as you apply it. Apply the compound to the wall in light, straight strokes (Image 2). The skim coat needs to dry for about 20 minutes before the walls can be sanded and painted.
With the ceiling plaster secured with plaster washers (and the severely damaged areas repaired by installing with drywall), and a bonding primer applied, a thin layer of gypsum compound can be applied. This will provide a smooth surface and help conceal the areas where the old paint was chipped (Image 1). Whether you're using gypsum compound or plaster, the application technique is similar: Apply a thin coat of compound with the wide blade (Image 2). Apply the compound in long, even strokes and filling in any low spots (Image 3). Then go back with a wide-blade scraper to level the surface (Image 4).
With the walls coated with a skim coat, and the coat dried, sand the surface using a sanding block. When sanding walls with existing horizontal or vertical seams, sand across the seam -- rather than directly right on top of it -- to avoid gouges or uneven spots. You'll know you've sanded sufficiently when you can run the palm of your hand across the seam, and you can't feel a difference from the rest of the smooth wall. Sanding the walls generates lots of extremely fine dust. Be sure to wear a dust mask when sanding.
After sanding, cover the freshly patched areas with new construction primer to seal the new gypsum compound. This is sometimes termed "spot priming" or "flash priming". You'll only need to apply this primer to the areas where gypsum compound or drywall compound has been applied.
Finally, after sanding of the walls and ceiling is complete, application of primer can begin. One application technique is to load the roller with paint, then roll a large "W" pattern on the wall, to spread the paint evenly across the surface. Then, go back and fill in the remaining areas for a solid coat. To save time and money, rather than using plain white primer, you can tint the primer to the same color as your finish-coat color. Once the primer has dried, the walls and ceiling can be given a fresh coat of paint.