Action

  When a string buzzes when you play it open, but all the fretted notes sound fine, the problem is probably a worn nut slot. Once a nut gets worn out it is usually (but not always, as with some vintage instruments) best to have it replaced by a professional luthier. Sometimes, however, you need it fixed NOW!
  To repair worn or chipped bone nut slots in an emergency, (without removing the nut from the guitar), I use a combination of baking powder and super glue. First, I place a piece of masking tape across the front and back of the nut, to act as a dam. Then I put some baking powder in the bottom of the nut slot or chipped area, CAREFULLY apply a drop of super glue (super glue can seriously damage your guitar's finish, or you, in a heartbeat), and let it harden for about 15 minutes; the glue and powder will combine to make a hard filler that matches the bone pretty well. Remove the tape, clean up any slop carefully (with a fine-cut file and 0000 steel wool), re-cut the slot to the correct depth, and you're ready to play.

  Another common cause of string buzz, or high action, is the truss rod being out of adjustment. Most truss rods ONLY work in the area from the nut to about the 5th or 6th fret. When the truss rod is too tight (backbowed), notes in the 1st position (near the nut) will buzz, but the buzz will decrease and disappear as you play up the neck. When the truss rod is not tight enough (bowed), the action may feel high, even with the bridge lowered, and notes higher up the neck may buzz.
  To check the truss rod adjustment, I fret the low "E" string at the 1st fret with my left hand, and at the same time, fret it at the 12th fret with my right hand, and look for a space between the bottom of the string and the top of the 4th fret. (I then repeat this procedure with the high "E" string; uneven spacing could indicate a twisted neck). What I like to see is a space of about .005" or less between fret and string, (that's about half the thickness of my high "E" string). If the string is laying ON the frets (and notes in the 1st position buzz when played), the neck is backbowed, and the rod needs to be loosened. If there is much space at all between the bottom of the string and the top of the 4th fret, the truss rod needs to be tightened.

  If the truss rod is properly adjusted and you still have a few notes that buzz, then you probably have either uneven or floating (loose) frets. Either way, you need to get the frets leveled and crowned, fixing any loose frets first (floating frets can't be leveled).