Synthetic binders (plastics) were mainly used after the Second World War either as building materials or as binders in the field of conservation and restoration. In contrast to inorganic materials, synthetic polymers on buildings are subject to different and more severe signs of ageing due to environmental influences, which reduces their durability compared to inorganic materials. The degradation of synthetic materials is primarily caused by the influence of electromagnetic radiation (especially UV radiation) with accompanying oxidation processes. Three case studies from the federal state of Brandenburg in Germany are presented. In the medieval church St. Mary in Frankfurt/Oder at a baroque burial site around 1700, wall paintings, stucco and metal plates were restored with various synthetic polymers. The artistic decoration of a facade in Cottbus in 1972 was carried out with the aid of polyvinyl acetate, and building materials were tempered with polyester resin for street art in Schwedt/Oder. All objects currently show serious damage. The causes of the damage were analyzed in detail and solutions for dealing with the objects to improve their durability were discussed.