MDF, which stands for medium density fibreboard, is used for many household applications such as shelves and speaker boxes. It is easier to work with than regular plywood or particle board. MDF panels are extremely heavy and come in varying sizes with 122 cm by 244 cm (4 feet by 8 feet) being a common size. Sealing MDF with polyurethane will protect it from water damage.
Sand the MDF with 150-grit sandpaper. Carefully go over the entire area that is going to be sealed in a circular motion to smooth out the MDF. This will allow for easier sealing. Blow off any remaining dust and debris from the MDF that is left over after sanding.
Apply a coat of solid white primer to the entire panel. Spray a wet coat of the primer on the MDF without leaving anything unprimed. Allow 30 minutes for it to dry. Touch to make sure it is completely dry before moving on.
Sand the entire primed area with 220-grit sandpaper. Complete this procedure in the same manner as the sanding from Step 1, going over the entire area in smooth circular motions and cleaning off all the remaining dust and particles once complete.
Spray on a second coat of primer and allow 30 minutes to dry once again. Sand down the surface in the same manner as the previous step, once the second coat has dried completely. Wipe off the particles and dust from the MDF panel.
Apply a coat of polyurethane to the MDF using a synthetic brush. Use smooth, even strokes and apply a top coat covering the entire area of the MDF panel. Allow 30-45 minutes for the polyurethane to properly dry and add a second coat in the same manner as the first for complete coverage. Allow time to dry upon completion.
The edges of the MDF will not seal like the face, even when sanded smooth. One option is to add a wood edging to the MDF. Another possibility is to add drywall joint compound around the edges and sand it down to ensure a consistent seal.