Ok so here what i've done - any for anyone esle painting white:
I've Sprayed them with Skull White Spray to base them.
then i watered down some Foundation Paint-Astronomican Grey, and coated the models with this light mixture. Once dryed i added a Lay of Skull White Piant and to add finishing touches i watered down some Chaos Black and lightly applyed that in specific areas - leg joints, vents etc.
all of that as given me a dirty looking white, which looks like my Models are Wartorn, and been in a few fights, but nor as it given me a gleaming white that looks almost "unreal". which is good.
on certain models i've topped them up with more Skull White, to show them alil cleaner. it does add a nice mix to models