Re: Locking tuners?
this is starting to sound really interesting. what grit sandpaper?
I usually start with 400, then finish it with 1500 so it's very smooth.
Use a movement
towards the tuners only (I'd written back and forth, don't know what I was thinking), you don't need to apply any extra pressure. Just keep some tension on the string so that it guides your movement, but not too much or it will probably ruin your sandpaper. You can't afford to chop the nut slot the wrong way, it might make it too wide and that's not good either.
Now, with this method you can do 2 things.
#2 is probably not optimal, but I've hacked my guitars away with it and got good results in playability with no added issues:
1) Widen the slots, which is very easy and will probably solve your tuning issues quickly.
2)
Deepen the slots. This would be probably better done by a qualified luthier, but I've deepened the nut slots on all my guitars with this method to bring the strings closer to the frets. You just gotta keep chopping down after finishing the widening, but
slow please, if you go too low you'll get massive fret buzz with the open strings. I did ruin two nuts in my Epiphone. Gotta learn somehow.
It improves the intonation on the lower frets and makes barre chords easier to play.
If you go too low, mix baking soda with superglue and fill the nut slot with it, let it dry for an hour and sand away again. It will be as hard as plastic. Just don't do it for friends this way, it's not professional, buy a new nut.
Hope this helps!