Class 1A is typically referred to as the thicker chemfilm.  When Class 1A coating is left unpainted it provides corrosion protection with no measurable dimensional change.  It also serves as an exceptional base for subsequent coating applications by improving adhesion of paint finish systems on aluminum alloys.  Coatings of this type are used on fuel tanks, tubing, and component structures where paint finishes are not required on interior surfaces but are required for exterior surfaces.