Thinner Any fluid medium used to thinning paint or varnishes. In oil painting turpentine and white spirits have been used. Linseed oil has also been applied to oil painting. As goes for acrylic, water has often been used. However, after a certain point of dilution, the pigmentation breaks up unevenly. For thinner glazes it is then often better to use fluid matte medium to achieve semi-transparency. If the thinning is in aid of extra drying time, a retarder fluid might also serve the purpose better.