Near-Infrared (NIR) reflectography is a well-established technique for painting diagnostics, offering a fundamental contribution to the conservation of paintings. Since the '80s it has been routinely applied to study the execution technique of the author, as well as the presence of pentimenti, retouches, integrations or underdrawing. In the last decades IR reflectography has been extended to the visible (VIS) spectral range, providing information about the pigment composition. Up to now the multispectral analysis is still applied at an experimental level, as the processing of the image set is not straightforward. Rarely multispectral VIS-IR application has been applied to frescos, probably due to the lack, in most cases, of a scattering background. In this work we present the results provided by a multispectral scanning device based on single sensor acquisition, working in the 380-2500 nm spectral range, that is a laboratory prototype specifically built for research-grade imaging. The technique have been applied on a mock up simulating a mural painting substrate where an underdrawing, made of either carbon or iron-gall ink, was covered by different surface layers of limewash, the so-called scialbo.