Not only does varnish protect paintings from dirt and dust, it can even out the painting’s final appearance with either a glossy or matt finish. Over the years, dirt and dust will stick to the varnish, rather than the painting.

Gloss oil-based varnish, polyurethane and Danish oil can last 10 or 20 years, though satin finishes and stains may fail sooner as pigments and flattening agents disable the driers. Water-based coatings and paints can also be viable longer than three years.