Tito,
Like Karin, I believe Michael Georges is also very informed about underpainting. You can see a recently posted underpainting in Unveilings for the Seasoned Artist (Chase & Payton),
http://forum.portraitartist.com/show...?threadid=1159
just in case you missed it.
There is so much great information in this Forum, I'm sure you will find what you need. I'm learning and being encouraged every day.
Gina