Wetli initially trained as a technical draughtsman, but then took an apprenticeship at a private graphic studio. From 1936 till 1946 he worked as a painter and draughtsman in Berne and Geneva. Meanwhile he travelled a lot in Europe, Africa and the USA. At the same time, he also produced posters and book illustrations. In 1947–48 he studied at the Académie de la Grande Chaumière in Paris. A gentlemen’s fashion firm in Olten employed him in 1949–56. After that he opened his own graphic studio. He also continued to paint, and was commissioned for murals by Swissair in Milan and for the Swiss Expo 1964 in Lausanne. Wetli illustrated and wrote a book about Greece. He designed posters for cultural and travel organizations, as well as for industry.