Finishing Drywall Joints: Fill Coat
The tape coat leveled off everything, and the next two coats will make the surfaces smooth.
You need to use wider taping knives for these coats, from 7 to 12 inches. You want to build the joints up a little in the middle and then feather them out smoothly.
And you want to apply the mud a little differently, too, with a little less pressure and a little more patience.
Mudding Joints on Flat Walls and Ceilings
Use your taping knife to put more mud on the joint. Then smooth it out with a stroke down each side, then one down the middle.
For the side strokes, put more pressure on the outside of the knife and let it ride a little high in the center. For the center stroke keep even pressure on the knife.
With factory joints, this coat should extend about two inches wider on each side than the tape coat. Butt-joints, if you recall, don't have the beveled edges that the factory joints do so the they'll tend to build up higher with each coat of mud. Because of this you'll have to feather them even farther than with the factory joints.
After this coat is done you should not be able to see the joint tape.
Screw and nail heads get covered with another layer of mud at this stage too. The mud from the first coat has probably shrunk a little so you just want to fill them in flush with the surface.
Inside corners are a little trickier than flat joints. Once you've feathered one side it's tough to work on the second side without disturbing the first.
One way to solve this is to use a corner knife. Professionals do a coat on one side of the corner, then wait for that to dry before doing the second side.