A thin dope film does not provide sufficient protection of the fabric from the elements, and early deterioration of the fabric may result. Ultraviolet light, which is invisible, combines with oxygen to form an oxidizing agent that attacks organic materials. The ultraviolet rays can be screened by adding pigments to the dope film and by adequately covering the fabric with the dope. Aluminum powder usually is added to two of the dope coats to stop any ultraviolet light from reaching the fabric. Undoped fabric or fabric covering that is not protected by coats of aluminum-pigmented dope should not be exposed to sunlight for long periods.
Adequate protection of the fabric usually is achieved if the dope film hides the weave of the fabric, leaving a smooth surface. This cannot be determined by the number of coats of dope applied, but rather by the dope film thickness. This varies with application technique, temperature, dope consistency, and equipment.