noun
paint thinner
A solvent used to dilute oil-based paints and clean brushes.
油絵の具を薄め液で薄める。
Thin oil paints with paint thinner.
薄め液は換気の良い場所で使ってください。
Use paint thinner in a well-ventilated area.
シンナー is a more general term for thinner, often associated with organic solvents and sometimes with substance abuse; 薄め液 specifically refers to paint thinner for artistic or DIY use.
Compound of 薄め (usume, 'dilution') + 液 (eki, 'liquid').