Finish Coat and Texture
The finish coat is where you have to be a real artist. You don't want to leave any grooves or streaks after you're done.
Before starting, scrape a wide knife over all the joints to smooth them out a little. This removes the ridges and tool marks. You want the base to be as smooth as possible for this final coat.
Finish Coat on Walls
When the joints are still dry, check to see if you have any large humps. Do this by holding the edge of a knife against them and rocking it back and forth. If you've got a large hump, you'll have to feather the joint more.
You can use the joint compound straight out of the can. Some professionals like to thin the mud out a little for the first coat and this last one. It's really a matter of personal preference.
If you do thin your mud, do it with only a cup of water at a time. And don't get it so runny that it falls off the knife. And keep in mind that thinning the mud too much will weaken it.
For this coat you should be using wide knives, about 8" for the screw and nail heads, and up to 12" for the joints. Use the same techniques as the last coat, only here you want to feather the joints as smoothly as possible.
Ceilings tend to be the most exposed part of a drywall job. Light thrown across it by ceiling fixtures really bring out any irregularities in the surface. Walls are usually broken up by furniture, windows, door or wall hangings, so their flaws are a lot less noticeable.
The most common way to treat a ceiling is with a texture to help hide any mistakes.
One treatment we've used is sometimes referred to as "knockdown." For this you need to rent a sprayer and use it to "splatter" thinned joint compound onto the ceiling. There are also special texturing compounds made just for this purpose. After the mud sets up a few minutes, you flattened it with a broad taping knife.
Some people like a more "pebble-like" appearance to the ceilings. For this type of effect, you mix part mud, part paint, and part aggregate, which are small vermiculite-like particles.
This type of texture doesn't get flattened out. But, as with the other texture, it'll still be necessary to scrape the over spray off the walls.
You can also get different ceiling effects by rolling the texture on instead of spraying it. You can even swirl, or "stipple" it with a stiff brush.
Ceiling Skim Coat
The moisture and heat in bathrooms and kitchens are more likely to cause dirt and stains on the ceilings. For these areas many people choose to go with a flat surface on the ceilings which is easier to clean.
This is called a "skim coat", and it's applied instead of a third coat as a smooth, thin layer of mud over the entire ceiling. Be aware that this is a challenging job for a novice taper.
First sand down the mudded areas of the ceiling. Apply this coat with a 10 or 12 inch wide trowel.
Work in as large of an area as you're comfortable with. Load up some mud and then spread it out over the area. Then go back and smooth it out. You actually end up taking off almost all the mud that you put on.