A British professor who specializes in cities and urban life has been convicted of damaging luxury cars with graffiti that was surprisingly polite.

Stephen Graham was found guilty Friday of using a screwdriver to scrawl inoffensive words such as "very silly," "really wrong" and "arbitrary" into the paintwork of vehicles including a Mercedes, an Audi, and a Volvo.

Prosecutors said the spree caused 18,000 pounds ($29,000) in damage.

Graham, who teaches at Newcastle University in northeast England, pleaded guilty to criminal damage at Newcastle Crown Court.

Graham's lawyer said the 47-year-old academic had no recollection of his August vandalism spree. Graham previously blamed a bad reaction to alcohol, antibiotics and prescription drugs for his behavior.

Judge James Gos said he would sentence Graham next month.