With professional quality framing & drywall that includes flat, solidly anchored drywall surfaces, the tape coat (bonding compound) is wiped flat and allowed to dry over all of the joints. Most tapers tend to cut corners when using all-purpose mud and double up on the tape coat to achieve a tape and first coat in one pass. All bonding compound in excess of that required to bound the tape, is then flushed out, leaving the tape flat as possible in the middle of a 4″-6″ wide grayish seam.
The first top coat includes regular light compound. The process involves starting with the butts and will go flush to the tape thickness. The mud is then built up on either side of the tape and the first butt top coat will be much wider than the recesses and angles, but all will be the same thickness. The mud is then pulled tight to the tape on the first top coat so it will dry the fastest. If the mud is hammered on and forms a hump, it will then require excessive time sanding, which can otherwise be avoided.
The second coat conceals the tape, with the butt and recess widths widened out and the edges feathered. If this is done right, the tape should be undetectable once complete. The second coat feathered to the left and right side, is then finessed down the middle on the skim down to hide the tape. If pulled too tight, the tape will reappear. This requires just enough finesse wipe down to cover. The angles can be finished in two top coats with a 6″ knife with prep in between to scrape away any high spots and make it level.
The third top coat is considered to be a skim coat, which fills in any valleys and fish eyes and then pulled tight to get a perfectly flat surface that makes it impossible to detect any abnormalities. The result should be completely flat, just like the original drywall sheets.
Finally, by sanding the recess joint width, the factory beveled edge ends up being about 12.″ Butts on good construction are 24″. and wider if framing is bad. The total depth of all the finished and sanded coats on butt and recesses is surprisingly small!
Drywall mud should be as thin as possible to uniformly cover the surface and no thicker.