Painting MDF

Almost at the blink of an eye solid wood such as pine, oak and poplar was replaced by MDF (medium density fiberboard) in the new home market and quick to follow in the DIY and remodeling market. MDF is an engineered wood product molded by breaking down softwood into a powder and combining it with wax and resin forming panels by applying high temperatures and pressure. This inexpensive substitute to the real McCoy has its disadvantages and advantages to solid wood.

Advantages:
Less expensive vs. real wood
Trim packages sold pre-primed
Disadvantages:
Messy cutting (dusty)
Very soft in comparison to even pine
Chips and dents easily
Absorbs moisture
Absorbs paint quickly
Inadequate factory applied primer
Shrinkage and swelling
Requires longer acclimation
Nail holes require alternative filling methods
Swelling may occur around nail holes after filling
MDF is marketed to builders as a cheap alternative to wood and the word “cheap” is all it takes for many builders to jump on the wagon. So what does this all mean for the paint contractor? In two words, “more work”. The professional paint contractors find MDF as a challenge for producing professional finishes, the other mainstream contractors see it as ” less work”.
Professional or not
Professional contractors must somehow for a lack of better words, ‘polish a turd’, as we like to say in the industry. The sole process of taking a piece of shit and turning into something it’s not. This is where the challenge comes in.
Mainstream contractors find working with MDF as less time on the job. MDF trim packages come pre-primed from the factory and all the contractor is required to do is fill the holes, caulk (or not) and apply a coat of paint, calling it done. Some methods for the mainstream contractor is spraying with an airless, brushing one coat and others go as far as simply applying the paint with a roller.
How to produce professional finishes on MDF
First things first, shop-vac the trim. Pay close attention to areas around nail holes where chips of primer may be stuck around the hole from the installation with a nail gun, sanding off or removing any protrusions on the surface.

The next three steps can be reversed or changed to fit your liking.
Fill nail holes with your choice of nail filler or putty, experiment for best results. Some guys use regular nail putty, others use a type of spackling and some use a combination of the two.

Re-prime everything with a slightly reducing quality oil-based primer. The goal here is to recoat the poor quality factory primer so the gloss paint finish will not disappear. A quality acrylic primer can be used as a subsitute for similar results but you are still at a disadvantage due to the fact the factory primer will absorb water-based primer and you may experience drag or heavy brush marks.
Caulk miters, door stops and all gaps even if the casing is tight to the jamb. MDF will shrink if proper acclimation did not occur.
Sand lightly and vacuum dust
Apply two coats of your finish product.
Observations when working with MDF
If you apply a coat of acrylic paint to MDF without priming first, you will notice your paint quickly absorbed into the factory primer and heavy drag, making it near impossible to achieve professional results.
Filling nail holes with putty is easier when filled after the new coat of primer. The fresh primer coat will help reduce putty from drying out and crumbling out of your hand. Also, the putty tends to break-off in the hole easier and cleaner after fresh primer vs. doing it over factory primer. You may experience shrinkage if the hole is not fully filled, pushing the putty twice firmly in the hole with help prevent shrinkage as the putty dries out.
Caulking trim after the first coat of finish will save your fingers. If you can caulk cleanly I suggest this method. Caulking over either the factory primer or your new primer will be abrasive not only to your finger but also the tip of the caulk tube.
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