Barnes has contributed artwork for his brother's band, of Montreal, including covers for Aldhils Arboretum, Satanic Panic in the Attic, The Sunlandic Twins, and False Priest. The covers typically have unique color schemes and illustrations achieved through a combination of thick watercolors, markers and ink. Barnes also contributed T-shirt, tote bag, and wall decal designs for the Skeletal Lamping Collection, which re-imagined the concept of album packaging by including digital album downloads in a variety of different art pieces.
In 2008 he designed the album artwork for Sand Fastened, the debut album from the band Fire Zuave, which features singing by a Chuck Andrews (a cousin of Barnes').
Barnes has also created art for Automaton Snowboards.
On April 26, 2011, Barnes released his debut book, "What's Weird," featuring 128 pages of paintings, drawings, sketches, and more. The book comes with a commentary by Barnes.
David also works on of Montreal's music videos, including "Gronlandic Edit.". Along with Jason Miller and Apollinaire Rave, he co-wrote and appears in the video for "Coquet Coquette," from of Montreal's 2010 album False Priest.
In addition to conceptualizing of Montreal's stage productions as well as designing their sets and props, David also often appears as a performer during the band's concerts. Along with of Montreal, he has performed on Late Show with David Letterman, Late Night with Jimmy Fallon, and Late Night with Conan O'Brien.
In 2005, Barnes worked with his brother and his sister-in-law, Nina, on A Pollinaire Rave, a touring comedy production.
In July 2008, Kevin and David performed skits, under the name Disney Herpes, opening for Fire Zuave in Athens, Georgia.