then a hard line will form at the bottom of your stroke, because the paint will start to dry. if your paper is not of artist quality then the hard line forms much faster.
what brush should i use? – you can use a round brush or a flat brush for the wash. make sure it can hold a lot of water. a mop brush works really well for wash as it holds the maximum amount of water.
what is the size of the brush i should use? – size of the brush really depends on the size of the area you are going to wash. a brush should hold enough water 