If painting on walls without politely asking is your thing, then Montana is not to be missed. The Barcelona shop for the spray paint company functions as a shop and gallery dedicated to all things graffiti, including (of course) spray cans, pens, art books and street wear.
Montana is in many ways at the heart of the Barcelona street art scene, so this could be your chance to chat up some practitioners. They also host regular exhibitions by people connected to the street art scene.
Get there for gallery openings for schmoozing and boozing - details on the website.