In the 1960s a group of Pop artists began to imitate the commercial printing techniques and subject matter of comic strips. The American painter Roy Lichtenstein became notorious for creating paintings inspired by Marvel comic strips and incorporating and enlarging the Ben Day dots used in newspaper printing - surrounding these with black outlines similar to those used to conceal imperfections in cheap newsprint. At the same time Andy Warhol was also using images from popular culture, including comic strips and advertising, which he repeatedly reproduced, row after row, on a single canvas until the image became blurred and faded. The German painter Sigmar Polke also manipulated the Ben Day dot, although, unlike the slick graphic designs of Lichtenstein, Polke's dots were splodges that looked like rogue accidents in the printing room. In a similar vein, Raymond Pettibon undermined the innocent spirit of the comic strip with his ink-splattered drawings and sardonic commentary.