The reason I mentioned was that after painting whole airframe with a relatively thick layers of paint which became stiff sometime later, and some of the rivet heads on the main spar at the wing root on the upper and the lower surface looked so much loosened, at least to the eye of our inspector. So we had had those fasteners replaced by an aircraft company at its factory nearby. They replaced the existing fasteners, each consisted of an outer sleeve and a rivet core, with modern type fastener system.