Sentence:  For the re-lining, the usual method is to strain a new and strong cloth of an even surface upon the stretcher, to rub it down smooth with pumice stone, and then to give it an even <WSD>couch</WSD> of paste, a similar <WSD>couch</WSD> is then to be applied to the back of the picture after it has been freed from all inequalities; ... 
Meaning of the word marked with <WSD> tags:  A preliminary layer, as of colour or size.