Scrape the wall thoroughly of all loose paint, dirt, crumbling plaster and anything else that might interfere with the adhesion of the thin coat, using a paint scraper.
Repair any deep cracks or holes by spreading on some joint compound with a 4-inch knife, pressing a piece of drywall tape over it, then spreading more compound over the tape. Get the compound smooth and flat. Let it dry for three or four hours.
Spread the first coat of thin coat over the wall starting at the top and working down with your 12-inch drywall knife. Work in horizontal courses, pulling the knife across the wall at a shallow angle, making the plaster thin enough that you can see through it on the flat spans of the wall. It should fill any small cracks and depressions. Do the whole wall.
Let the compound dry about four hours. Scrape over it with your 12-inch knife to knock off any ridges or peaks.
Lay on the second coat in the same manner as the first, starting at the top and working across and down. Again, keep it very thin, almost transparent, so it fills the depressions without piling on the wall.
Let the second coat dry for four hours. Scrape it, and add the third coat in the same manner.
Let the third coat set for eight hours. Sand it lightly, by hand, just enough to take off any lines or ridges and smooth it all out. Coat the wall with sealing primer.