MDF is an inexpensive composite wood product formed from wood fibres and resin bonded under heat and pressure. It is used more than any other product for kitchen cabinets, where it is often veneered, but it has many other uses, many of them requiring only a paint, oil stain or urethane finish.
Finish raw and unprimed MDF with an oil-based stain and then a high-quality floor wax. An architect friend who develops low-cost housing often uses MDF rather than hardwood on floors. This is a stunning finish that looks expensive but isn't. One word of caution: Any time you are finishing MDF with a transparent product, do not sand the MDF before application. The result will inevitably be blotchy. Also, never apply MDF directly over floor joists; it will sag. Always apply over a subfloor.
Finish MDF with an oil-based paint. While you sometimes see recommendations for a water-based paint on MDF, an oil-based paint will penetrate the MDF without dissolving the bond between wood fibre and resin and will seal it better. Usually two coats will suffice. It is unnecessary and inadvisable to sand the MDF before applying the first coat. After the first coat dries, sand it lightly, then apply the second coat. If there are areas where the second coat is still penetrating into the MDF (these areas will have a matt finish), apply a third coat.
Apply an oil-stain, with or without a finishing wax, on MDF used for cabinets. Some cautions apply: Do not use an oil-based stain alone in areas that will be exposed to water, such as under a sink. Over time water will penetrate the MDF and it will disintegrate. After these problem areas are oil stained, apply a high-quality urethane. If you don't like the gloss of many urethane finishes, a semigloss and a matt finish are available.
Finish MDF with a clear urethane. A gloss finish is most durable, but both semigloss and matt urethanes are available and will wear well.
Apply an oil-stain first, then use a clear urethane to finish the entire cabinet. Be sure to let the oil-stain dry thoroughly so that the oil-stain and the urethane don't interact. You can also varnish MDF, although the rationale for varnishing rather than oil-staining, its increased durability, is less compelling today than before the widespread availability of urethanes. In most cases where durability is an issue a urethane will have a similar appearance and will last longer.
Apply urethane, paint or varnish to small areas with a brush. Cover larger areas with a roller; however, if you use a roller with urethane to cover larger areas, such as floors, it will leave small bubbles that will dry into the medium. These are relatively inconspicuous. For the highest-quality finish, spray urethane or apply it with a special-purpose applicator available at most home-improvement stores. Sometimes these can be rented. The applicator is basically a large wiper that comes with its own tray, and it will ensure that the urethane does not bubble when applied.
If you are planning to stain MDF, be sure to stain some sample panels first. MDF takes stain quite differently from hardwoods. Experimenting with the amount of colourant in the stain by hand rather than buying an off-the-shelf stain with colour will be worth the extra effort. These colourants, in a wide variety of colours, are available almost anyplace that sells paint. Dark stain treatments on MDF–black-infused reds and greens, for example–are particularly effective.
Urethane and oil-based paints and stains should only be used in well-ventilated spaces. If you are finishing a floor, wear a chemical mask (available at home-improvement stores) and keep doors open. These are potentially toxic substances. Don't put used paint rags in a closed space (like a garbage can) until they have thoroughly dried. Wet paint rags can spontaneously combust. More than a few home workshops have caught fire when this caution wasn't observed.