One of the ways that people use to determine how much restoration a painting has undergone and it's possible affect on the value of the painting is by viewing the image under ultraviolet light. The mediums used in restoration vs. the original paint layers fluoresce differently when viewed under UV. At Lowy, we tend to opt for the "less is more" philosophy -- we see a lot of things that don't actually need restoration, but it has been done anyway, for what could be a variety of reasons. For example, if there's a flake loss in the paint layers, in some cases it may be hard to match not just the color, but the brush stroke or the impasto. It may be easier to match if a larger area than necessary is painted over, or "overpainted", instead of carefully dotting in the missing paint, or "inpainting". It's always better to do less than more, so as to be as minimally invasive to the painting as possible, and so we always take the time to inpaint everything, regardless how difficult the match.
A lot of paintings that come to Lowy for an evaluation have previous restoration which is overdone. We recently had a painting come in that had drying crackle and some structural paint losses which had been drastically over-restored and overpainted. Selective cleaning tests confirmed our suspicions that much of the overpaint seen under UV was unnecessary and inappropriate. We removed the overpaint and re-did the restoration, and the results were dazzling -- take a look at the fluorescence before and after! This is just a small piece of the overall image, but you get the idea. The second is another UV photograph before and after of a project we worked on here at Lowy.