Most underbase grays and athletic white inks respond well to smoothing stations. This station may be as simple as a fully exposed solid screen that is printed in the cooling station after a flash.
The heat from the flash units, along with the pressure of the squeegee mimics a transfer press. The result is a smooth surface for subsequent colors to be printed on top. Other options for a smoothing station would be a "hot head" iron or a heated roller.
Source: Ray Smith