Im building the 12-string tele, so hopefully the tension of the strings will fix some of the backbow. but the backbow is so big, when i put my straightedge (ive notched one side of a 50cm piece of machincrafted metal) on the neck and put it flush at the nut there is a 3mm gap at "the end of the neck" (after the 21th fret) and a 1 mm gap between the 12th and 13th fret.
You guys think that the tension will straighten all of that?