Exterior MDF

      A closer look at exterior-grade MDF products. October 27, 2008

I have a customer who wants some arches built of wood to go over some exterior windows of her house. I have thought about building them with MDO because they paint up well. Does MDO paint well on the edges same as the face? If not, is there something to apply to the edges before painting? Exterior veneer edgebanding? Is there an alternate wood for exterior that has good face and edges for painting like an exterior MDF?

Forum Responses
(Architectural Woodworking Forum)
From contributor J:
MDO does not paint well on the edges and edgebanding will not work well - too much movement.

Extira is an exterior MDF product. I've used it a few times and it seems to hold up very well. Check with your supplier about how best to paint it.

From contributor S:
I took a sample of Extira and put it in a glass of water and sat it on my desk for a week. When I took it out I didn't notice any flaring along the edges and it was the same thickness.

From contributor J:
I took the sample my supplier gave me and threw it in my side yard for more than a year! Through sun, rain and four seasons it hasn't moved a bit.

From the original questioner:
Thanks for all the replies. This sounds like just what I need, however I am from Toronto, Canada and can't seem to find this product anywhere. Is this a fairly new product? Is there an alternative to Extira that I might be able to get in Canada?

From contributor V:
I am in Edmonton and I believe they have it in Winnipeg as well.

From contributor O:
I have a lot of experience with Extira. It is a hardwood composite with zinc borate added. If and when bugs eat it, they die. So, here ya go...

MDF, Medium Density Fiberboard. This is sawdust mixed with glue and a mat of about 12" thick is pressed between to steel plates (platens). These platens may be heated to set the glue. Compression can generate enough heat to set the glue also. The platens are open on the sides and the squeeze-out gets trimmed off.

MRMDF, Moisture Resistant Medium Density Fiberboard, one trade name is Medex. At one time this was considered exterior rated and used extensively by the sign industry. The binder is an exterior rated glue that will withstand exposure to moisture better than urea formaldehyde. With changes in manufacturing and failures in the field with exterior use, it is not listed as exterior any longer. It is used a lot in bathrooms and such. As with MDF, the platens are not contained and the squeeze-out is trimmed. The platens used to squish the MRMDF mat are heated to set the binder and the result is a hard exterior shell with a softer middle. I like to use this when I need a stronger or more rigid MDF product.

Exterior MDF, this is the best term for Extira. It is a hardwood composite. Hardwood chips (rough sawdust) and zinc borate are mixed with the binder. In this case they use a phenolic resin.

The forming of the material is similar to other composite products. Except the platens that are used to squish the Extira are contained on the sides. This creates a very uniform consistency. Also steam is injected into the mat to set the resin. This resin is not softened with exposure to moisture. They sand one side of the material and it is very consistent in thickness.

I do not work for them. I did a lot of research into their product while creating Victorian millwork for exterior elements. The biggest problem with it is how slippery the dust is on the floor of the shop. I had one architect run it through her dishwasher 3 times to see how it would perform. For the record, I am not an engineer and probably have some, if not all of my facts screwed up.

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