When does an unwanted hard edge commonly form in watercolour?

Correct answer: Wet colour stops at the boundary of a dry area

Watercolour edges are controlled by the moisture relationship between adjacent areas. When wet paint reaches a dry boundary, its movement stops there and the pigment settles along the line, producing a sharp unwanted edge. Equally wet areas allow softer spreading, transparency concerns pigment appearance rather than edge formation, and a soft brush alone does not create a hard boundary.