We experimentally demonstrate that electromagnetically thin polyimide substrates can mitigate substrate-induced detrimental effects to the performance of metallic metasurfaces. A planar quarter-wave plate for the microwave K-band is fabricated on a polyimide substrate of deep subwavelength thickness by means of standard photolithography. By properly selecting the combination of the polyimide thickness and the aluminum layer thickness of the metasurface, conversion from linear to circular polarization is achieved at the design frequency. The proposed approach is generic, and it can be applied to the fabrication of mechanically robust, flexible metallic metasurfaces, which are primarily designed to work in a free-standing configuration.