Hello!
A Filter is a painting technique, similar to a wash. I use MIG productions filters, although it's too easy do it. You need oils and solvent (ej. turpentine). Mix both in 1/3 proportion (1 of oil / 3 of solvent). After, you must use a plane brush only wet (removes the excess on a paper), and apply brushstrokes in the same direction.
A wash is with 1:1 proportion (oil + solvent) and you must use a plane brush very soaked.
I hope you understand me!, anyway, ask me!.
Braxandur: I dont know the mean of "rust", I thought that rust = oxide :S!
Cyu!