A coating apparatus, for applying a coating substance to a paper web, has a chamber from which the coating substance is supplied via a slit type mouth communicating with an applicator space which is defined by the paper web and a doctor blade element. At the web, on the side of the applicator space from where the paper web enters, a throttling passage is formed by a back-pressure element in such a manner that a stream of excess coating substance issues from the applicator space via the throttling passage and abruptly separates from the web and in so doing flows away from the web predominantly as a film thereby preventing completely or almost completely the admission of air into the applicator space by the air boundary layer adjacent the moving web.