A synthetic route is demonstrated to build purely polymeric nanostructured multilayer coatings, adaptable to arbitrary surfaces, and capable of efficiently blocking by reflection a targeted and tunable ultraviolet (UV) range. Reflection properties are determined by optical interference between UV light beams reflected at the interfaces between polystyrene layers of different porosity and hence refractive index. As no dopant absorber intervenes in the shielding effect, polymer degradation effects are prevented. Alternated porosity results from the modulation of photochemical effects at the few tens of nanometers length scale, combined with the collective osmotic shock induced during the processing of the precursor diblock copolymer film. Experimental evidence of the application of this method to coat rough surfaces with smooth and conformal UV protecting films is provided.