My paint film is showing heavy brush marks after drying. What's the cause of this?

Coarse or heavy brush marks are most likely caused either by painting onto a very porous surface, or applying paint in warm conditions. To get rid of the marks, first clean down the surfaces to remove all dirt, grease and surface contaminants. Rub down with Wet & Dry abrasion, using water or a suitable solvent. Finally, rinse down and allow to dry thoroughly before applying a new coat of paint.