Apparatus (1) for injecting a liquid in an area of a thermal spray gun (100). The apparatus (1) includes an injector cleaning device (10) having an inlet connectable (19) to at least one feedstock supply line (18), an inlet connectable (17) to at least one gas supply line (16), and an inlet connectable (21) to at least one liquid medium supply line (20). An injector (30) orifice is coupled to the injector cleaning device (10) and is adapted to at least one of inject a liquid jet into a hot stream created in the area of the thermal spray gun (100) and receive feedstock, gas and liquid passing into the inlets.