How to Prepare a Door for Painting
Brush primer on the door.
Use a 2-1/2-inch brush to apply the primer. Let it dry thoroughly.
Smooth away any imperfections using 180-grit sandpaper. If you notice tiny imperfections in the primer that will show through on the finished layer, lightly sand the primer after it dries using 180-grit sandpaper. Be sure to wipe the door down with either a damp rag or a tack cloth.