Basically trying to prepare myself a dossier for painting techniques.

I've got a few areas on models I'd like to pain white eg. interior of cloaks, Gandalf the White etc.

I've got 2 Minas Tirith banner bearers, one being the guy with the large, more elaborate banner with his sword out. I was planning to do a scheme on this with a white banner for this dude.

After a black undercoat is it just reundercoating the banner white and then going from there? Also are there any techniques for white fabric like this?

Also in models like Gandalf the White, is it worth re-undercoating certain areas such as straps, Glamdring, face... black for consistency?

Personally I always start with a grey base-coat for white. And I build up to white so that the creases are well shaded, for whites these shadows are very important to make it look dynamic.

Also when I'm painting a model like Gandalf I do undercoat the skin areas black, helps create contrast and helps deepen the look of skin

_________________
Commission Painting @FB http://www.facebook.com/squyrepainting
Commission Customers include:
GBHL Youtube Channel
MiniWargaming

Personally i always undercoat white, it gives a brighter finish, if that is what you want.
If I am painting a white area I tend to work in reverse. I paint the area white and then add the shading using progressively darker shades.

_________________
Commission Service Available

mallysminiatures.blogspot.co.uk

When I want to paint white cloth I gradually build up the color from a light brown (Graveyard Earth using GW's old paints); it is followed by Karnak Stone, and then Bleached Bone. Depending on the model I may use more or less gradations.
But my final step is a mixture of white with Bleached Bone; I almost never use pure white, it is too lurid.