ORIGINAL: Campgems
The less sealing surface, the better. Do some math. Figure the area of a 0.002" wide band around the valve and figure the pressure sealing the valve, figure 50 PSI of compression for a bench mark and and in the couple lbs of valve spring. See what the sealing PSI is. Now do the same math for 0.006" band. There will be quite a difference. Even a minimual use of abrasive will accomplish this amount of band increase very quickly. As the band increases in width, there is an accompanying requirement for more pressure to maintain the same seal. Seeing as our pressure is a fixed element, keeping the band narrow is the way to get the best seal.

Don