I'm putting together a slabside upper and an A1 upper with triangualr hand-guards.  I have a couple of sets of hand-guards that are decent which I plan to use.  But I also have a beat-up pair that I've been things about sanding smooth, etc.

If I cleaned the current finish well (say ending at 400 or 600 grit), what would be the next steps?

TIA,

Marv_45

If you wet sanded down to 600, you may find they look fine if buffed a bit.  

Matte black Alumahyde II from Brownells works well.
Set the can in warm water for a few minutes. It sprays better warm. Degrease item with alcohol. Apply 3 or 4 very thin coats about 5 minutes apart. Set aside for at least a week to cure.
BTDT with exactly your situation and am pleased with the result. Works equally well on pistol grips and buttstocks.
FWIW
YMMV