To skim coat plaster walls, mix the setting compound with water, and wipe the wall with a damp cloth. Scoop the compound onto a trowel, and apply the compound using smooth horizontal strokes. Then, hold the trowel at a 30-degree angle to scrape away excess compound and smooth the surface. Working in small sections, complete the wall, wait one hour, and sand the wall with 120-grit sandpaper. Repeat the process using vertical strokes.
When mixing the setting compound, add enough water to achieve a thick soup consistency that allows the compound to spread easily while remaining in a mound on the trowel. Adjust the mixture with water or additional compound if necessary.
Occasionally, dip the trowel in a bucket of clean water, and wipe the edge of the tool to clean it. After applying compound to a small section of the wall, scrape the compound on the trowel back into the bucket with a joint knife. Use a fresh scoop of compound to continue skimming.
Avoid scraping the wall too much, as this can cause bruising. Instead, allow the compound to set, and sand it to remove imperfections. Apply several thin coats of compound if the wall isn't smooth after the second coat, sanding between each coat. Allow the compound to cure for 24 hours before painting it.