How can dark paint accumulation at the edges be avoided while filling a flat background?

Correct answer: By lifting excess paint in time with a clean damp brush

In a flat wash, wet paint can collect at a boundary and dry as a darker edge or back-run. The useful corrective principle is to remove the excess while it is still wet, using a clean damp brush and a controlled stroke. Adding water may spread the pool, rubbing can damage the surface, and pausing at the boundary increases accumulation.