Dr. Wood T-121 T-121 Paint Thinner  (1 L)

Paint thinners are diluents and usually refers to a solvent that is used to dilute or "thin" oil-based paints[1][2] — and similar things, such as varnish, which in essence are a colloidal suspension of a solid dispersed in a liquid medium — to reduce the viscosity and so making them 'more liquid'. Typically, this happens after the paint has lost some of the solvent while in storage, so have become too thick and dried-out for use.