yes, of course bristles - some brushes are bad for that. What bothers me most when painting with acrylics are lacquered brush handles where the lacquer flakes off when you leave brushes in water (I do until I get around to cleaning them). But the razor blade treatment is championed by Brigid Marlin to get a perfect glass-clear surface between glazes, i.e. flattening out any impasto strokes also.