Skim coating is the process of applying plasterboard mud to a wall or ceiling to smooth its surface. A wall or ceiling can be skim coated to cover cracks, imperfections or a previously-applied texture. The mud is applied in multiple coats and fills in low spots until a flat, smooth surface is achieved. For ceiling applications, the existing texture, usually a popcorn finish, must first be scraped off before a skim coat can be applied.
Climb the ladder and use the 20 cm (8 inch) plasterboard knife to scrape off existing texture such as popcorn. Hold the knife at a shallow angle and push the edge away from you to scrape.
Use the 20 cm (8 inch) plasterboard knife to load some plasterboard mud into the mud pan. Dip the plasterboard knife into the pan and scoop some mud onto the knife.
Start at one corner of the ceiling and apply mud to the ceiling with the plasterboard knife. Apply the mud while holding the mud knife at approximately a 30 degree angle. Apply light pressure to the knife as you move it across the ceiling. Repeat this process until the entire ceiling is coated.
Wait approximately 24 hours to allow the mud to dry. Use the plasterboard knife to gently scrape off overlap ridges and high spots.
Use the 30 cm (12 inch) plasterboard knife and apply another coat of plasterboard mud to the ceiling, continuing to fill in the low spots and cracks. Wait another 24 hours until the second coat dries. Repeat this process until all low spots and cracks are filled.
Wait 24 hours for the final coat to cure, then attach a fine plasterboard sanding screen to the pad on the plasterboard sanding pole and sand the ceiling. Use long, even strokes and avoid sanding in one spot for too long. Keep sanding until the surface is smooth with no high spots, low spots, cracks or texture.
Lay down a tarpaulin to catch plasterboard mud drips and plasterboard dust during sanding if you are working in a room with carpet or furniture.