I haven't found an efficient way to remove hard oxide from valves. What I do, is use a rotary wire brush, but stay away from the part of the stem that slides in the guide. The wire brush removes the carbon and other chippable materials, leaving something that is valve-shaped, but may have black areas. I ignore those, and get on with the lapping. (Of course the seating surface, on both valve and seat, must be completely free of black after lapping.) Incidentally I apply the wire brush quite firmly - valves are not delicate when you are using a tool softer than they are - but do not touch them with a grinding wheel.
As soon as you begin lapping, you will see whether you are getting full-circle contact on both valve and seat. Once you have a full circle, you are lapping to make it wide enough. If it is wider in some places than others, the correct procedure is to re-cut the seat and/or have the valves ground in a proper valve grinding machine (depending on whether it is valve or seat that has the varying contact width). Permissible seat width for both inlet and exhaust is 0.8 to 2.0 mm. About 1-1.2 mm sounds excellent, so long as it is continuous all the way around - do not tolerate any radial lines or cloudy patches.
From what you have said, it appears that the stems and guides are in very good shape and you can proceed to lapping. Remember when you are lapping be very, very careful that no grinding paste gets into the valve guide, or you will ruin it immediately. Just keep the paste onto the seating area, by checking it and cleaning it off frequently. Use a separate cloth for wiping the grinding paste - do not allow it to touch any other part of the machine, and throw it away as soon as you finish the lapping job.
