Apr 14, 2007

Painting MDF

painting mdf trim crownmold

A close look at how painting MDF trim can look like it was sprayed using a paint brush.

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 article was UPDATED on FEBRUARY 21, 2012

Skip the mumbo-jumbo Jack and tell me HOW TO PAINT MDF

This inexpensive substitute for real wood has advantages and disadvantages.

Advantages

  • Less expensive vs. real wood
  • Trim packages sold pre-primed

 

Disadvantages

  • Factory primer is the weakest link, chips easily
  • Messy cutting (dusty)
  • Very soft in comparison to even pine
  • Dents easily (some more than others)
  • 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 trim

MDF is marketed to builders and remodelers 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 paint contractors? In two words, “more work”. Professional paint contractors find MDF more of a challenge for producing professional finishes, mainstream contractors see it as ” less work”. We have five different types of MDF in our market which further complicates the process unless a system is developed to paint MDF regardless where it came from.


Professional or not

Professional paint contractors must somehow turn a product barely ready for paint into an acceptable painted surface. This is where the challenge comes in.

Some paint contractors view pre-primed MDF trim = less time on the job. MDF trim packages come pre-primed from the factory and all the paint contractor is required to do is fill the holes, caulk (or not) and apply a coat of paint, calling it done. Some paint contractors spray trim paint directly over the pre-primed MDF with an airless sprayer or brushing one coat of paint, while others simply applying the paint with a roller.


How to paint professional hand-brushed finishes on MDF trim

First things first, shop-vac the trim. Pay close attention to areas around nail holes where chips of primer or glue strands may be stuck around the hole from the installation with a nail gun, sand off or remove any protrusions on the surface. If the areas around the nail holes are puckered—they need to be sanded out and leveled prior to the steps below.

 painting mdf trim

BEST METHOD

The first three steps can be reversed or changed to fit your preference but they are listed how we prefer. This method produces very nice results but it is highly dependent on the oil-base primer coat. You are on your own if you skip STEP 1

  1. Re-prime MDF with a slightly reduced quality oil-based primer, my preference is Zinsser Cover Stain. The goal here is to recoat the poor quality factory primer to provide a) gloss retention, b) better surface to fill and patch nail-holes, c) a slightly abrasive surface. 
  2. Fill nail holes with your choice of nail filler, patch or nail 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. We use a combination of Color Putty Brand Nail Putty for curved profiles and lightweight patching compound for flat surface areas applied by a flexible putty knife.painting MDF trim
  3. Sand patches lightly and vacuum dusty filmpainting MDF trim
  4. Caulk miters, door stops and all gaps even if the casing is tight to the jamb.
  5. Apply the first coat of trim paint. We used BEHR ULTRA Semi-Glosspainting MDF trim
  6. Sand any brushmarks out of the 1st coat finish. Anything seen now before the 2nd coat will also be seen through the 2nd coat of paint. Here is our sanded 1st coat.painting MDF trim
  7. Shop-vac all sanding dust
  8. Apply the 2nd coat of trim paintpainting MDF trim

 

MDF can shrink as much as 1/4” if proper acclimation did not occur prior to installation.

 

*A quality acrylic primer that retains gloss can be used as a substitute 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 with some trim paints.

More about STEP 1

Zinsser Cover Stain oil-base primer is available off-the-shelf ready to spray. We reduce the stock primer by 8 oz of thinner to a gal. You may want to only reduce half a gal at a time to ensure the most workability. Keep the lid closed on product not in use. We have successfully reduced the primer by more than 8 oz /per gal. A greater reduced primer coat still adheres better than any acrylics known to me.

Observations when working with MDF

If acrylic paint is applied to MDF without re-priming first, you will notice paint quickly absorbs into the factory primer creating drag because the paint is drying too fast, making it near impossible to achieve professional results without leaving brush marks all throughout the MDF.

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 will help prevent shrinkage as the putty dries out.

Caulking trim after the first coat of finish will save your fingers.  I suggest that method if you can caulk neatly. You may want to glance over your work before you apply your 2nd coat of trim paint. Caulking factory primer or a new primer can be abrasive to your fingers and the tip of the caulk tube.

7 comments:

John said...

I'm a painting contractor, i have some experience with mdf. I have used lacquer primer mostly. Can i use something cheaper? like kilz?

Jack Pauhl said...

Kilz 2 is a good product for MDF so is Zinsser PRIMECOAT2. Both are very inexpensive solutions.

Better results can be achieved for semigloss finishes using ICI Gripper. Any primer that will not absorb your semigloss paint. ICI Gripper has a low sheen finish, its not dead flat.

whitemw said...

Any help? I have Trim MDF that is factory primed. I sprayed latex semigloss on the MDF. I went back to sand a couple paint runs, the latex has adherred to the primer and the paint and primer is just peeling off the mdf like plastic leaving bare mdf.

Any idea why this is happening?

Thanks.

Jack said...

whitemw, that issue you are experiencing is fairly common. It does not happen with all MDF but we have experienced it ourselves. This is obviously a factory primer adhesion issue when the primer pulls off exposing bare MDF. Nothing you can do about it to prevent it from happening. In your scenario, the factory primer is the weakest link.

Dimmi said...

I want to repaint the mdf trim that's installed and already painted (looks like satin or semi gloss). Do I follow the same procedure you describe for factory finished mdf? Which primer should I use? And do I have to reduce the oil based primer? I'm painting with HD ultra semi gloss.

Also I have mdf in the bathroom that I want to replace. What trim material should I replace it with?

Thanks

Leslie said...

Any recommendations for what type of product to use as a primer for raw MDF (not factory primed)?

And then what type of paint to use after primed?

Thanks!

DAH124 said...

Hi- I know this is an old thread but I was wondering if you could help me. My home builder painted my mdf utility room cabinets and they look very striated. It looks like paint drips running the length of the cabinet. They said it was due to the mdf but I know that it is not because the mdf that was installed was completely smooth without any imperfections. They used a spray gun and originally painted it white and then a charcoal color. Would I be able to sand them down and then have them resprayed? Thank you.