A polarizing sheet based on a fully metallic structure is here reported for the first time. The polarizer consists of a doubly-periodic arrangement of squared-wave guide sections enclosed by two perforated screens. Though the waveguide sections are highly reactive (operating below cutoff), the perforations placed at both sides of the unit-cell allow for total transmission at a certain frequency band. Within this band, the unit-cell allows additionally to tune its anisotropy in order to transform the linear polarization of an impinging 45 degrees-slant plane wave into circular. An equivalent circuit model of this structure is proposed that reproduces its full-wave scattering properties and explains its principle of operation. A design is performed operating around 20 GHz showing good performance.