A good clear coat should at least double the outdoor lifetime of a billboard. This is a compound that neutralizes the effects of any light that gets thru the UV barrier. Besides protecting the ink from ultraviolet light, a good clear coat also provides a strong physical barrier against wind and rain. Wind carries fine particles of sand and grit that literally sandblast away the ink. Without any clear coat protection, the ink will be directly exposed to airborne abrasion - and will eventually be worn down to nothing.
Raindrops carry air pollutants.
When raindrops dry on the surface of a billboard, the pollutants are concentrated - and can directly react with unprotected ink. Just as important as weather protection, is the fact that clear coat greatly enhances the appearance of a billboard. After all, the dollar value of a board is based on how it looks. Clear coat makes any outdoor advertising look significantly better. Without clear coat, a printed surface is not truly smooth.