Acryl is formulated for easy airbrushing, but you can brush it. Be sure the model is clean, use good brushes, and use good technique, brushing from wet to dry, only brushing once, and flowing on the paint. The brush needs to glide over the surface, if it drags, it needs more paint. It may help to use an acrylic flow enhancer to slow the drying, since that contributes to brush mark and coverage problems. And frankly, enamels are more user-friendly for handbrushing.