The weathering resistance of organic materials has been substantially increased by protecting their surface with a photocured coating containing both a UV absorber (UVA) and a radical scavenger (HALS). A kinetic study by real-time IR spectroscopy has shown that HALS have no effect on the cure rate, whereas UV absorbers slow down the cure process, due to their radiation inner filter effect. 3D analysis of the depth of cure profile revealed an incomplete cure at the coating/substrate interface, leading to adhesion failure. To prevent this detrimental effect, the UV-cured coating was photochemically grafted onto the substrate. The polymer material was first coated with a thin layer of a benzophenone solution in a diacrylate monomer. Polymer radicals, generated by hydrogen abstraction from the substrate by the excited benzophenone molecules, effectively initiate the polymerization of the acrylate functions, thus ensuring a chemical bonding between the coating and the substrate. The grafting reaction was characterized by ATR spectroscopy analysis and by surface energy measurements. Excellent adhesion was achieved by applying to the treated substrate a photocurable acrylate coating, containing the light stabilizers, because of the copolymerization reaction taking place with the unreacted acrylate double bonds of the base coat, upon UV exposure. The efficiency of this on-line stabilization process has been demonstrated on poly(vinyl chloride) that was made eight times more resistant to accelerated weathering.