For those of you who don't understand how beautiful the technique of underpainting and glazing can be, I'd like to invite you to see Scott Bartner's important demo here (his finished painting shown below):
http://forum.portraitartist.com/show...2&page=1&pp=10
Properly done, glazes do not "weaken" paint. You can look at any Vermeer or DaVinci and see how these delicate glazes have withstood the test of time.
Scott, do you teach?