The most surface preperation you will want to do is to wash your model thouroughly, and make sure there are no imperfections on the kit, as these WILL stand out like a sore thumb.
Secondly, make sure there are no bits of dust or hair on the area yu are about to cover with foil, as these will also stand out.
Take a small piece of foil and cover a part of your model that has obvious dust bits etc on it, this will allow you to see and judge just how clean your kit has to be.
Hope this helps