(edited - I just realized you are talking about the tangs)
One way is to file them in small groups, filing lengthwise along the fretboard, not one at a time.
That way the tang ends support the file off the fretboard edge until they are right down to the
surface. Once the file just begins to mark the surface, you will be about as flush as you are going to
get. If the edge has a finish on it, you have to avoid marking it up any more than you are able to
polish out later.
And for the fret ends themselves, (original post):
You file them at an angle, so the file is never actual flat against the side of the fretboard.
You do have to be careful not to go any deeper than the fretboard edge. You could put
some masking tape along the edge, just to protect against slips. But you need to file the
fret ends right down flush with the wood if you really want it smooth.
Then just polish up the fretboard edge afterward.
Last edited by megafiddle : 11-25-2012 at 10:31 PM.