Most modern vehicle covers, boat and biminis tops have soft tops made of durable polyacrylic / polyester canvas or clear vinyl fabric. Clear vinyl rear windows will absorb a wide range of gases and liquids, including water, acids and hydrocarbons. Since the clear vinyl has the ability to absorb foreign materials, it will yellow overtime, if it is not properly maintained. Very often, this discoloration is the result of ultraviolet (UV) light cross-linking, i.e. the clear vinyl is not directly affected by the sun’s direct UV rays, but its absorbents can be.