Try Cowdisley. Informative, serious.
Another good forum:
I agree about Cennini - some crazy shit.
Unfortunately, there are a few of those out there.. Take a peruse through GoodArt. It's like Pravda.
But there are excellent forums out there. Places where serious traditional visual art, drawing and painting techniques and materials etc. can be intelligently discussed in an mature, intellectual environment and moderated by serious supporters.