Working with MDF Mouldings

Tips on using MDF mouldings, with comments on coping, gluing, and fastening. April 8, 2008

In a few weeks our company will be using over 5,000 feet of MDF mouldings. The size range is 3 to 8 inches and includes everything from base to crown. We are looking for installation tips: what to do and what not to do, especially regarding inside corners and nailing.

Forum Responses
(Cabinet and Millwork Installation Forum)
From contributor J:
In most cases, just treat it like real wood. Cope inside corners, wood glue at joints, etc. There are a few things to be more careful about. MDF will split more easily if you try to pin nail casing miters. Copes are a bit more fragile and that thin edge will break, crush, dent more easily. Be careful when you are springing in a coped piece... don't make it too long. The learning curve is pretty easy. Your guys will get the nuances of MDF pretty quickly. Personally, I try to avoid MDF moulding.

From contributor A:
Not many like changes, especially an old timer, however... Pick up that MDF glue out there - it helps out pretty good. The only thing I profited by with the MDF was I made a Murphy bed in my shop and I made the face of it MDF raised panel and painted it white. Solved a lot of problems, cost, expansion/contraction as opposed to any other species, didn't have to worry about hardware screws, etc since they didn't apply. Otherwise follow the previous post, and also try and stay away from it, because most of the time it just isn't worth it in my old opinion.

From the original questioner:
In the first two responses some of the reservations we had with using MDF are coming to the front. Typically we use poplar for paint grade trim but our millwork salesman suggested MDF for two reasons, price per foot and size of moulding. I must admit that he has a point, but now I am wondering if at the end of the job it will be worth it? Guess we will find out.

From contributor S:
I don't want this to get into a cope/miter thread, but wanted to point out a factor. MDF comes pre-primed so a glued cope to a primed face will not bond well. At mitered corners you want to glue both pieces because the MDF will wick up the glue and starve the joint. At copes use caulk to bond and fill joint. There are some other nuances as already posted. Once you get the hang of it you will be very happy with speed of install and net loss factors. A lot of labor will be saved with less scarf joints, not needing to cut around defects, and using all straight boards.

From contributor K:
Try a search on MDF trim in the "search this site" box in the top right corner of the page for some good discussion, pro and con.

1. MDF doesn't like to get face-nailed. 15 ga nails especially will leave blow-out welts, which often have to be cut off with a razor blade after the first coat of primer.

2. Sand the pre-primed stuff very carefully, especially when touching up outside crown corners and the like. The primer is often harder than the surface of the MDF, leaving a ridge; cheap or coarse sandpapers will leave fuzz. Both will frustrate the painter coming in behind you.

3. You can get away with a little less scribing on MDF crown when the ceiling presents bulges and bellies because it flexes a bit, especially on long runs.

4. At the same time, if you pre-glue and assemble your mends, as many do, that flexing 26 foot length of MDF is a lot harder to manage than a 26 foot length of poplar, especially 20 feet up.

5. Besides, scribing MDF is a pain unless you use a sanding wheel on a grinder Ė you canít plane it, for instance.

From contributor J:
I've never had a big problem with nailing MDF. Sanding outside corners can be tricky... use a higher grit to avoid the fuzz. When I install crown I snap lines to keep it straight, regardless of the dips in the wall or ceiling. Long pieces of MDF moulding can be a bit hard to handle. You can plane MDF. To get good results you need to back cut the stock first so the edge to be planed is thinner.

From the original questioner:
Thanks for your responses. I followed the advice to search the site on MDF. There are a lot of opinions, especially on glue. Any ideas as to which is best: regular wood glue, molding and trim glue, or 2p-10?

From contributor G:
For glue I would use an adhesive caulk or maybe even Fastcap 2P-10. I've use TBII before and wonder if it works that well, seeing how MDF soaks up glue like crazy - the joints are still holding up, though. For coping, I found a rotary tool with a small sanding drum on it works well, just be careful. As for nailing, I've never had good luck firing 16ga. into the thinner parts of the moulding. I found it easiest to install a continuous backer so I can nail through the thickest part of the moulding - works very well, and you don't need to worry about finding studs. Like previous posts, use a pinner at the outside miters.

From contributor S:
At joints, I spread glue on both surfaces twice with my finger.