Durable Coating for an MDF Product

      A manufacturer seeks a prime coat that will protect his MDF pieces from dings during handling or shipping. March 29, 2006

We are making a MDF product that has routed edges and funky shapes and will be painted different colors, so we can not give it the protection it needs before shipping from the shop to local businesses. If much time is spent on packaging, we may not be able to compete with others. These pieces may be handled more than once and we need a way to coat them with something that will make them a little more durable - not necessarily indestructible, but reduce the damage factor. I've heard of powder coating, and spraying water and glue combo, and just a thick primer. Any ideas on the most cost effective method?

Forum Responses
(Finishing Forum)
From contributor M:
If they are going to get painted white, I'd go with a good brand of pre-catalyzed or post-catalyzed lacquer primer. The catalyst in either causes the paint molecules to chemically bond and form a very durable finish. Pre-cats are ready to shoot right out of the can but don't quite compare to the post-cats (do not be fooled, though - certain brands are very good). Post cats have a liquid catalyst that is added to the primer just before using (you have eight hours to use, so it's not a rush). Most are a 10% catalyst mix, easy to do. For MDF, and depending on the level of "pretty" you want, sand the routed areas with sandpaper. I personally use either a worn out 320/400 grit or a new 600 grit. You should polish the routed edges to feel like Corian before you shoot them, or the raw MDF will fuzz like an electrified cat (ugly!). Wear a respirator and have a good exhaust hood, please, as these products are hard on your lungs.

Following are the products I use in my shop and would recommend for you, both made by ML Campbell. I also recommend using an airless gun (not air assisted, if it's legal in your area) since they can flat out lay some paint in a hurry.

Magnaclaw - a pre-cat primer. Our normal choice. Good stuff. Dried to sandable finish in about 30-45 minutes. Two coats is plenty.

Clawlock - the post-cat primer. Higher solids than above, tougher, easier to sand. You may eek by on one coat with this stuff if you sand the routs smooth enough. Requires Care Catalyst.

From contributor C:
If I were in your position, I'd experiment with using Bullseye dewaxed shellac as a sanding sealer. It would be just one coat followed by a light sanding. This will allow a very wide range of topcoating treatments and minimize grain raising (MDF fiber swelling). You'd need to work a few samples to assure that the system does what you need and to achieve an effective and efficient procedure. It might also be good to ask one or two of your customers to try out the resulting products and give you some feedback on how it works for them. Glues are sometimes used (the white glues... Elmers, etc.), but really do not have the same level of performance that shellac does as an undercoat, and they are water based and certain to cause more fiber swelling than shellac (which means more sanding afterward).

From contributor D:
With the dewaxed shellac in mind, is there a water based shellac that will not cause swelling of MDF? I use water based products except when working with MDF. Usually it is primed and painted, but I would like to experiment with pigmented lacquers to get a better finish.

From Paul Snyder, forum technical advisor:
If you're looking for a durable paint, then you should consider a pigmented conversion varnish or catalyzed polyurethane. Here's a link to a chart that briefly compares the durability of various spray finishes.

Related article: Spray Finishes

It is possible to get a durable finish from a quality acrylic (aka latex/water-base) or oil-base enamel paint that you'll find at your local paint store, but the cure time is longer than the catalyzed finishes designed for spraying. And... house paints are made to be brushed or rolled, though they do spray well with an airless sprayer. Do you have any local finishing suppliers in your area (e.g., Chemcraft, Becker Acroma, ML Campbell, Sherwin Williams Chemical Coating store, etc.)? If so, any one of them can set you up with a durable primer and pigmented topcoat. They should be able to mix any color you want.

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