Abstract: An evaporative coating apparatus in which a coating liquid drains under gravity from the bottom of a processing tank through a metering valve and a pipe into a supply tank. The apparatus includes a supply tank for providing a supply of coating liquid; a processing tank for receiving the object to be coated having (1) an outlet for the coating liquid positioned in the lower part of the processing tank, and (2) at least one wall shaped to provide a substantially parabolic relationship between the surface area of the coating liquid in the processing tank and the hydrostatic head between the coating liquid in the processing tank and the coating liquid in the supply tank, whereby the level of the coating liquid in the processing tank decreases at a substantially constant rate; and a means for passing the coating liquid from the outlet in the processing tank to the supply tank.