What kind of compound is best to use when skim coating a plaster wall that had wallpaper removed. Should it be thinned with water? when would one use Toping Compound instead of joint compound. Light weight compound, blue bucket, or regular, green bucket if going with joint compound
I've used regular joint compound, thinned with water quite a bit. Be careful with any areas where the wallpaper pulled up the outer layer of drywall and exposed the brown paper - those will tend to bubble up when you skim over them. You might have some success with painting over those areas with something like Kilz, but even then, you might get bubbling. In that case, I use a utility knife to cut out the offending bubbles, and skim again. Thin coats should help reduce instances of bubbling.
General purpose compound will work fine. Be sure to read the label as some manufacturers specificaly state "Not for Skim Applications".
As said by the others, thin with water and mix well with a large drill and paddle to remove as many air bubbles as possible before attempting to apply. You can also add a few ounces of Ivory dish soap to the mix. It will really make the mud glide on smoothly with fewer tiny pock holes. I prefer a good 12 inch knife to a trowel, and dipping it in water before final pass helps make a smoother surface. If you do need to put on a second coat, which is almost always the case, be sure to knock down any trails or ridges with a drywall knife and lightly sand the surface to remove boogers and loose dried mud. clean the entire surface with a damn sponge, rinsing the sponge often. This will remove all the dust that will cause the skim mud to curl off the wall during application.
Once you are happy with the final finish, be sure to prime using a PVA primer or other primer specificaly described to be used on new drywall. What you are doing is sealing the mud so the water in your latex paint won't be absorbed into the mud, lifting it and ruining all your hard work. Good luck.
This suggests joint compound but still not sure if there is something else that might be better for some reason: http://www.thisoldhouse.com/toh/asktoh/question/0,,455195,00.html