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Medium density fiberboard, or MDF, is a composite wood product similar to particleboard. It's made out of wood waste fibers glued together with resin, heat, and pressure. MDF is appropriate for many applications, from cabinetry to moulding, because it is smooth, uniform, and won't warp.
MDF has many advantages over plank wood, particleboard, or high density fiberboard. It's very smooth because the wood fibers used in its manufacture are uniform and fine. This makes it have low "tear out," which means that when sawed, the end has a smooth cut instead of a jagged edge. This also means that a coat of primer and a couple of coats of paint take well, leaving an attractive, finished surface unlike other composite wood products. MDF also has a mild reaction to moisture, meaning it won't warp or swell in high-humidity applications like a bathroom cabinet.
Builders use MDF in many capacities, such as in furniture, shelving, laminate flooring, decorative moulding, and doors. They value MDF for its insular qualities in sound and heat. Also, it can be nailed, glued, screwed, stapled, or attached with dowels, making it as versatile as plank wood.
The smooth and stable surfaces of MDF and the well compacted machined edges are prerequisites for successful finishing with a wide range of liquid coatings. Pigmented lacquers are widely used to create strong single colour effects or, using modified lacquers and a good finishing technique, unique pearl, metallic, marble or other dramatic effects. Alternatively, the natural appearance of the MDF surface can be enhanced using a transparent stain with a clear lacquer topcoat.
The surfaces to be finished should be free from dust or sanding marks. MDF, normally supplied with a 100/120 grit or better surface, will be suitable for most matt finishing treatments without further sanding. An additional light sanding with 150/180 grit is recommended when using high gloss finishes or where a minimum coating thickness is required.
As the edges of MDF are more absorbent than the surfaces, finishes applied to the edges may differ in appearance from the surface finish. In particular, the increased absorption of stain into the edges will result in a darker colour compared with the surface. With lacquers, a higher coating weight may be required on the edges to accommodate this increased absorption.
In order to control the problem, lacquer manufacturers have developed specially formulated sealers, usually two pack, at a high solids content for application to the MDF edges before finishing in the normal manner.
Stain and clear lacquer
As an alternative to obliterating the MDF with pigmented lacquer, the appearance of the finished product can be enhanced by taking advantage of the variable absorption of a stain applied directly to the surface to highlight the fibre structure.
Solvent borne stains applied directly to the MDF surface will wet the surface effectively and ensure an even colour. Water borne stains can also be used but the waxes sometimes added to fibreboards to improve their water resistance may result in uneven absorption of the stain and a consequent variation of colour.
The stained surface can be protected by one or two coats of clear lacquer with a light denib with 320 grit paper between coats. Deeper colour effects without obliterating the character of the MDF surfaces can be achieved by lightly tinting the lacquer used for the top coat.
High gloss finishes
The high stability of MDF surfaces and edges can be used to good effect for the production of gloss finishes using a high build coating based on polyester resin. At one time, a polyester finish applied by spraying would need to be mechanically burnished to achieve a high gloss surface free from imperfections. Using suitably formulated lacquers, high gloss finishes can now be obtained directly from the spray gun without a subsequent burnishing treatment. As a further development some companies are now applying a polyester base coat to develop the colour on MDF with a clear lacquer top coat to protect the surface and enhance the gloss effect. Non yellowing clear lacquers must be used on white and light colour base coats to maintain the colour.