INERT:
Chemically inactive; resistant to corrosion.

INORGANIC:
Matter other than that of animal or vegetable origin. For example, minerals and simple salts are inorganic materials.

INSOLUBLE:
The inability to be dissolved.

INTERCOAT:
A layer of paint that is "sandwiched" between two others. Also refers to something occurring between coats, as in "intercoat adhesion."