Section 211.275  Aerospace Flexible Primer

"Aerospace flexible primer" means a primer for aerospace use that meets flexibility requirements such as those needed for adhesive bond-primed fastener heads or on surfaces expected to contain fuel.  The aerospace flexible coating is required because it provides a compatible, flexible substrate over bonded sheet rubber and rubber-type coatings, as well as a flexible bridge between the fasteners, skin, and skin-to-skin joints on outer aircraft skins.  This flexible bridge allows more topcoat flexibility around fasteners and decreases the chance of the topcoat cracking around the fasteners. The result is better corrosion resistance.

(Source:  Added at 45 Ill. Reg. 3509, effective March 4, 2021)