To prevent brush strokes when painting with a brush first avoid pressing down too hard on the brush when painting and apply a thin coat of paint first without stretching the paint on the brush.
Then after the first thinner coat of paint dries then paint again using another coat of paint and only paint with the paint brush in one direction.
Sometimes you may have to use 180 grit sandpaper and sand smooth the brush strokes and then paint over it again or use a sponge paint brush to avoid the brush strokes from a regular bristle paint brush.