First make sure your paint is thin enough to flow, but not enough to run. Don't brush over the same spot too often, with Acrylic, only once or twice is good. At first it may seem you see the brush strokes, but with practice, you will see much of the strokes will disappear as the paint dries. However, if you keep going over it in an effort to remove the strokes, thats where the uneven drying patterns will show up as stroke marks. Finally, keep a wet edge, meaning each stroke to partly go over another recent wet stroke. Don't follow a with a second coat until the 1st coat is totally dried - sometimes 2 coats will be the only way to completely cover the plastic.