The relative importance of two key optical parameters, surface roughness and refractive index (RI), were considered in relation to the effects of a varnish on the appearance of an oil painting. An experiment was performed in which a cleaned painting with high surface roughness was simulated by a photographic test target in optical contact with glass that had been sandblasted on one surface. Varnishes applied to the glass reduced the surface roughness depending on their molecular weight. Colorimetric and spatial analyses via a modulation transfer function were performed on digitized images of the test target. A computational analysis was also performed to quantify the role of RI. Both analyses indicated that the dominant optical parameter was a varnish’s ability to level a roughened paint surface. A varnish’s RI has, at most, a just perceptible effect. -- SCMRE