Wiessner, often in partnership with fellow immigrant Hans Kraus, established numerous first ascents in the Gunks, including many climbs that are popular (and intimidating) to this day. Perhaps their best known combined effort is the very popular "High Exposure" buttress 5.6, which they first climbed in 1941 with a hemp rope and three soft iron pitons. Other notable Wiessner first ascents in the Gunks include: "Gargoyle" 5.5, "High Traverse" 5.5, "White Pillar" 5.7, "Baby" 5.6, "Frogs Head" 5.6; "Gelsa" 5.4, "High Corner" 5.7 and "Yellow Ridge" 5.7. In 1946, he led "Minnie Belle", the first 5.8 in the Gunks.