The crowning file should be curved enough. I then use some wet 'n' dry paper (dry) to remove the file marks (normally P320 to P400 depending on how deep any scratches are, then go up the grades of Micromesh (1500 to 12000) to really get the fret smooth.

I use an offcut of top E string to drag along the top of the fret when using the sandpaper. If there are any scratches left, you'll feel the string pulling as it passes over. Once nothing happens, then start on the Micromesh. Alternately if you haven't got Micromesh pads or cloth, run through the grades to sandpaper up to P2000 or P2500, then rub the frets with some 0000 wire wool. This is a bit messier and the wire wool dust gets everywhere, so tape over pickups etc.

It never hurts to then use some metal polish on the frets.

You'll find the frets look a lot more rounded once they are polished. They can look a bit flat on the top when just filed.