A machine for processing a flat workpiece or wafer has a head, which spins the wafer between upper and lower weirs in a base. The head is spaced apart from the base by an air gap. A vacuum source attached to an exhaust opening in the base draw air through the air gap, to reduce or eliminate movement of process chemicals into the head. Corrosion of head components by process chemicals is reduced. The disadvantages of having a mechanical seal between the base and the head are also eliminated.