The surfaces of moisture resistant and exterior grade MDF are sufficiently smooth for painting without prior preparation. All machined edges should be cleared of loose dust and protruding fibres by sanding with 120 or finer grit paper. Sharp edges and corners should be slightly bevelled to ensure a satisfactory coating thickness on these vulnerable areas.
For the best results, all finishing materials should be obtained as a complete system from one supplier. Most coatings will be built up with a primer followed by basecoat and one or two topcoats. Particular attention should be given to the application of primer to all exposed edges and any face mouldings as the edges of MDF are more porous than the surfaces. All surfaces and edges should receive the full finishing treatment.
Several different exterior durability coating systems are available for use on moisture resistant or exterior grade MDF including masonry and non porous high gloss paints. Microporous paints are not considered to be suitable for finishing moisture resistant or exterior grade MDF.
Plaster type finishes can be applied to moisture resistant and exterior grade MDF subject to pre-sealing with an appropriate sealer.
Companies using moisture resistant or exterior grade MDF for outside applications in exposed situations should discuss the suitability of a particular grade of MDF and the selected finishing system with the materials suppliers.