Aghhhhhhhh just nearly ruined a painting, had a nice graded wash but wanted to intensify the colour without leaving a hard edge, area dry 24 hours then wet the area and dropped the colour in, the stronger end of the initial wash started a backwash. I have gotten away with this technique, say 90% of the time but now and again particulary with dark colour it bites me, perhaps I should give up washing, phew!!!!