Efficacy of Colour Protectors in Urban Art Paintings under Different Conditions: From a Real Mural to the Laboratory

Contemporary muralism is one of the most widespread manifestations of urban art in cities nowadays. Loss of colour (fading) due to degradation of the pictorial materials is one of the main manifestations of deterioration in this type of artwork. However, the application of protective coatings is considered to slow down fading. In this study, two acrylic-based protective coatings were applied to selected areas of a real mural painted with six different alkyd paints and one acrylic paint. The impact of the protective coatings on the original paint colour and the long-term efficacy were evaluated. For purposes of comparison, the same protective coatings were applied to paint mock-ups prepared under controlled conditions and exposed to an outdoor environment with marine influence and to an ultraviolet aging test. At the end of the exposure periods, physical and chemical changes were evaluated using a multi-analytical approach based on stereomicroscopy, Fourier transform infrared spectroscopy and scanning electron microscopy. Both protective coatings modified the original colour of the paints, with the impact being greater in the mural than in the mock-ups. The efficacy of the coatings as colour protectors is more appreciable in the most colour-stable paints. Thus, neither of the coatings provided good protection for the paints most sensitive to deterioration. The protective efficacy of the coatings was also influenced by their chemical composition and affinity for the painted surfaces.