(n.) One who thins, or makes thinner.

anticoagulant chromogen coat coat of paint coating color color filter color gelatin colorant coloring dead-color diluent dilutant dissolvent dissolving agent distemper drier dye dyestuff exterior paint flat coat flat wash floor enamel flux ground interior paint liquefacient liquefier medium opaque color paint pigment prime coat primer priming resolutive resolvent solvent stain tempera tinction tincture transparent color turpentine turps undercoat undercoating vehicle wash wash coat