Thought about this and came up with a bench made tool.
Take an old car exhaust valve, cut a slit in it about 1/4 inch deep. Go to your favorite machine supply shop, or Harbor Freight (save the jeers for later) and get 3 of these carbide cutter inserts 3 for 9 bucks.
put into slit and make 45 parallel to valve face about 1mm above face, tack weld or solder into place.
Lower it into your guide and chuck a variable speed cordless drill on the valve stem. 
Slowly turn in cutting direction while putting finger on valve cup to vary pressure. Use cutting oil and keep checking progress. Hey, it works! 
Make 2 more at 30 and 60 (or if your really good with this stuff, put the other 2 angles in the same valve) and do your own 3 angle, just watch your seat width. Check with good valve and prussian blue for contact area. What have you got to lose, 9 bucks?
Take an old car exhaust valve, cut a slit in it about 1/4 inch deep. Go to your favorite machine supply shop, or Harbor Freight (save the jeers for later) and get 3 of these carbide cutter inserts 3 for 9 bucks.
put into slit and make 45 parallel to valve face about 1mm above face, tack weld or solder into place.
Lower it into your guide and chuck a variable speed cordless drill on the valve stem. Slowly turn in cutting direction while putting finger on valve cup to vary pressure. Use cutting oil and keep checking progress. Hey, it works! Make 2 more at 30 and 60 (or if your really good with this stuff, put the other 2 angles in the same valve) and do your own 3 angle, just watch your seat width. Check with good valve and prussian blue for contact area. What have you got to lose, 9 bucks?
