A layer of varnish on a finished painting will protect it from dust, dirt, grease and UV rays. It will also even out any unevenness of sheen on a paint surface e.g. a gloss varnish will make the whole picture surface equally glossy. Picture varnishes should be removable so that they can be taken off and replaced with a new layer if the old layer gets damaged or dirty. Varnishes can be applied with a spray or a thin flat varnish brush, on to paintings that have completely dried. This can be 3-6 months after the completion of a painting. Oil varnishes can be used on acrylic paintings, but it is not advisable to use acrylic varnishes on oil paintings.
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