The first step to prepare MDF for painting is to sand it. Sand the surface and edges using an orbital sander with 180-grit sandpaper. Wipe away as much dust as you can with a dry cloth. Then use a damp cloth to make sure all the dust is removed. Wear the appropriate safety equipment when sanding.
Put a drop sheet down and place the MDF on the saw horses. Stir the undercoat thoroughly, then pour it out into a paint tray. Using a paint roller, evenly apply the oil-based undercoat across the MDF. Paint the edges using a paintbrush or roller. Let it dry.
Once the first undercoat has dried, give it a light sand with a fine-grit sandpaper. Wipe away any dust, then apply the second undercoat and let it dry.
Once the second undercoat has dried and been sanded, it's time to paint the MDF. Use the roller to apply the first coat of paint to the surface and edges. Let it dry and then apply the second coat of paint.
Once the second coat has dried, your project is complete and ready to use.