Sentence:  A straight-edge is placed upon the chalk lines, with the edge next the line slightly raised, and the brush, well filled with colour, drawn along it, just touching the wall, the pressure being never increased, and the brush refilled whenever it is near failing; but great care must be taken that it be not too full, as in that case it will be apt to <WSD>blotch</WSD> the line, or drop the colour upon the lower portions of the wall. 
Meaning of the word marked with <WSD> tags:  To mark with blotches.