There is no question that MDF makes a great paintable surface but how durable will the product be as wall base molding and door case molding? Is it going to chip out the first time the dog slides into it or the kids slam the toy truck into it? Thanks for the comments.
(Cabinet and Millwork Installation Forum)
From contributor A:
I'm not sure about chipping, but I worked on a home with all MDF moldings. My job was replacing the very poorly hung entry doors with new ones and adding storm doors. The original doors had leaked water during every rain storm. The base and casing at the bottom corners of the doors had swelled and discolored. I think I could have salvaged a wood trim and repainted in the same situation.
You can seal MDF by using a mixture of Titebond and water. I have made a couple of exterior carved signs out of Extira. It's great stuff, but you have to use oil based primers (it's waterproof you know). It's also fuzzy as a mule.
Just like any product, there are positives and negatives to using it. It cuts and mills easily, but is hard on tooling. Primed surfaces paint very well, but cut and exposed edges do not paint as well, even after sealing with glue or primer. The dust is ferocious, especially when routing. The ultralight mouldings are just that, very light weight. The sheet stock is quite dense and very heavy. None of the MDF has any real structural integrity, but for trim and moldings, it is just fine.
MDF has undergone a lot of changes over the last few years. We are probably on 2nd or 3rd generation MDF now so a lot of the bugs have been worked out. Many who used it when it first came out compared it to Masonite, a really awful product, and still refuse to use it. MDF comes in 3 types - Ultralight, Medium density, and High density.
Most of the standard profile moldings are made from Ultralight. It is soft, light weight, and very flexible. Nails easily penetrate it and usually don't pucker the material. Sharp edges can chip fairly easily if not handled carefully, but are fairly stable once nailed/glued in place. This is also the type that tends to swell the most when continuously exposed to water.
Medium density comes in sheet stock and nominal 1x stock. We use this for stair risers/skirts, mantels, and non load bearing columns. This type is much more stable and does not expand/contract as much as wood does. But, because of the density, the material tends to pucker when nailed. Some distributors purchase sheets in 16ft lengths from the manufacturer and re-mill it into moldings with their own profiles. But because of the many nails needed for nailing base and casing, the puckering problem becomes an issue and people refuse to use it and think all MDF is this way.
High density MDF is used for cabinet doors, countertop substrate, and anywhere a durable smooth surface is needed. It is very stable, very dense, and very heavy. This stuff is so dense that if you try nailing it with a nail gun, the nails will sometimes curl up and not penetrate. And if the nail does penetrate, you will most assuredly get puckering.
Bottom line - MDF has earned its place in the building industry and is definitely is here to stay. After working with it for awhile, you learn what it can and can't do as well as how to work with it. I have become accustomed to working with it and actually prefer it over wood for most applications. But it is not the be all, end all product. Something else will come along in the future and we will be saying, remember when we used to have real MDF to work with?
Comment from contributor H:
MDF is a builders dream, it allows them to squeeze every last penny out of a home. I work on homes that are owned by people who also own homes in Martha’s Vineyard, Nantucket, and the Hamptons. The only instance in which MDF has ever been used or spec'd by an architect is with custom designed paneling, or large surface areas that need the stability that sheet MDF offers over wood. MDF has never been used in these homes as trim.
As a 20 year professional carpenter, and a business owner I have seen the decline of the home building industry, by way of cheaper products such as MDF, which do nothing but unjustly rob the customer of their hard earned equity in the name of profits for so-called "custom builders" (first time home or not).
I would never recommend the use of this product as trim and would be wary of anyone claiming to be a high end or custom builder that does use it. I am all about being green, so why not re-use some of the trim from the old homes these money hungry builders are tearing down to put up cookie cutter homes.