MDF Molding or Medium Density Fiber Wood is an ideal cheap and eco-friendly alternative to wood. It is made of compressed hard and softwood fibers combined with resin to produce a grain-less material. Finishing MDF moldings is easy, the level of effort depends on whether you are finishing your crown or base moldings. Base moldings require greater effort as they are likely to have to withstand greater stress and cleaning than crown moldings; therefore a good finish will determine the durability of your molding.
Priming and Preparing the Surface
Apply a primer, use a high solid acrylic primer. Any other primer runs the risk of soaking into the MDF and might cause a little swelling. Once you have primed the MDF take time to get rid of any fibers that will be left… you can do this using fine-grit sandpaper or just a plain household scrubbing pad. Make note if your MDF needs sanding before priming, use 200 grit sandpaper and go over the moldings.
Painting the Surface Using a Brush
Apply two coats of paint for a perfect finish. Use semi-gloss or high sheen enamel paint for durability rather than a matte finish. You can paint your moldings before installation that way you will save a lot of time. If you are using a brush on 3.5-inch molding, go for a brush that has a 2.5-inch head, this way the finish will be less grainy. The grainy texture depends on the quality of the paint, brush, and technique used. Go for flat long strokes in a single direction.
Sanding Between Coats
Use fine-grit sandpaper and sand lightly the dried molding you just painting. Wait for it to completely dry out before you sand it. This will give the paint job a very professional look.
Applying the Second Coat
Apply the second coat either before application or after. It’s easier to completely finish your moldings before application and then cover up the scratches and holes where it has been nailed afterward. Or you can install the molding and then finish it completely; it is a matter of preference and convenience.
To fill out the holes made in your MDF molding during application with caulk which is the same color as the paint, use the same caulk to fill out corners as well. Then sand it a little and paint it over if needed. You can finish with a urethane varnish for added gloss, this again is optional, and this will make the molding more water-resistant. Always remember that moldings get a lot of battering so always focus on durability.