In 1937 Thrash joined the government-sponsored Works Progress Administration (WPA)'s Federal Art Project. Through the WPA, Thrash began working at the Fine Print Workshop of Philadelphia. At the Fine Print Workshop of Philadelphia, Thrash, along with Michael J. Gallagher and Hugh Mesibov, began experimenting and co-inventing the process of carborundum mezzotint, a printmaking technique. Carborundum printmaking uses a carbon-based abrasive to burnish copper plates creating an image that can produce a print in tones ranging from pale gray to deep black. The method is similar to the more difficult and complicated mezzotint process developed in the 17th century. He used this as his primary medium for much of his career and created his greatest works with it. One of his first pieces employing this nascent technique was his anonymous self-portrait entitled "Mr. X".