Lightly sand the MDF door with 120 to 150 grit sandpaper. Press lightly only to remove any dirt, debris and grime that may be on the door since MDF comes already sanded and ready to paint. Sand in the direction of the grain. Be cautious around the raised panel edge so you do not damage it.
Brush away the dust with a rag. Along the raised edge, use a paint brush to remove the dust.
Clean the MDF with rag moistened with warm, soapy water to remove any remaining dust. Do not saturate the door, especially near the edges of the raised panel where there could be gaps that the water can seep into. After the door dries, it is ready to paint.