Ok, for all those Pyramid heads who want a good way to make your helmet (or anything else) look rusted (you need to spray paint black as a base). What you need is:
Burnt Sienna or Red oxide universal pigment(Available at hardware or paint stores)
Polyacrylic clear top (also available at aforementioned stores)
Spray bottle
Denatured Alcohol (also avaiable at aforementioned stores)
Mix a little pigment and clear top together, then apply with a decent sized brush and spray with denatured alcohol. This will make it run and bubble and make areas dry transparent.
This guy did it and his helmet is sick.
http://www.instructables.com/id/Pyramid_Head_costume/
He explains the procedure more in depth in the comments. Have fun!