A friend of mine came to me with a Gibson 335 with a severe up-bow. The "relief" is 0.9mm (0.035") at the most without strings and with the truss rod loose. Even with fairly soft strings and a tightened truss rod, the relief is more than acceptable, and with my friend's super heavy wires the guitar is not playable at all. I think that the fret board must be absolutely straight without any tension in either way to be able to fight against these strings. Is there a better solution than sanding the fretboard down these 0.9mm? Removing the fretboard to remove the relief from the neck itself is out of the question. I am also skeptical to hammer in overdimentional fret tangs to straighten the neck as I find too tight slots difficult to handle.