From the late 1930s to the early 1940s, Sprang continued to work as a freelance illustrator, primarily for such pulp magazines as "Popular Detective", "Popular Western", "Phantom Detective", "G-Men", "Detective Novels Magazine", "Crack Detective" and "Black Hood Detective"/"Hooded Detective", for which last he also wrote some stories. Between 1937 and 1938, Sprang provided assistance on the King Features Syndicate comic strips "Secret Agent X-9" (layouts) and "The Lone Ranger" (pencil assists). In 1938, he also wrote briefly for the "Lone Ranger" radio series.