Watercolor painting has the reputation of being quite demanding; it is more accurate to say that watercolor techniques are unique to watercolor. Unlike oil or acrylic painting, where the paints essentially stay where they are put and dry more or less in the form they are applied, water is an active and complex partner in the watercolor painting process, changing both the absorbency and shape of the paper when it is wet and the outlines and appearance of the paint as it dries. The difficulty in watercolor painting is almost entirely in learning how to anticipate and leverage the behavior of water, rather than attempting to control or dominate it.
Many difficulties occur because watercolor paints do not have high hiding power, so previous efforts cannot simply be painted over; and the paper support is both absorbent and delicate, so the paints cannot simply be scraped off, like oil paint from a canvas, but must be laboriously (and often only partially) lifted by rewetting and blotting. This often induces in student painters a pronounced and inhibiting anxiety about making an irreversible mistake. Watercolor has a longstanding association with drawing or engraving, and the common procedure to curtail such mistakes is to make a precise, faint outline drawing in pencil of the subject to be painted, to use small brushes, and to paint limited areas of the painting only after all adjacent paint areas have completely dried.
source(http://en.wikipedia.org/wiki/Watercolor_painting)done by you xing
