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What are the differences between Medium Density Fiberboard (MDF) and Medium Density Overlay (MDO)?

In woodworking circles, MDF seems to be much maligned, yet MDO seems to be rather highly praised. Based on the names, at least, it seems that they are, at the core, essentially the same thing. Does the "overlay" really make such a difference that it takes a mediocre-at-best product and elevate it to the top of the pile?

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    Just to mention, you may not have these where you are (or it may be a special-order item) but here in Europe there are better grades of MDF. Way better. The first is what we call MR-MDF, the MR standing for what you might guess, moisture-resistant. The best of it is acceptable for exterior applications under cover. There are also some actually weatherproof MDFs made in a couple of countries, so good that some need no finish; these are typically available in a large range of thicknesses, and in a range of colours (including yellow and red!) and are through-coloured so edges and faces match.
    – Graphus
    Oct 22 '21 at 8:01
  • The resemblance between MDF and MDO stops at the name; the two products are about as similar as medium density foam and medium density polyethylene.
    – Caleb
    Nov 3 '21 at 6:20
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MDF is Medium Density Fiberboard and it's made by essentially combining resins with sawdust and compressing it into a fairly dense, extremely flat and smooth sheet. In general it can be worked like "regular" wood but it's heavier (because of the density) and tends to not hold screws as well as "real" wood. It isn't very pretty and has no grain but it can be painted well.

MDF isn't really a "mediocre" product, but it has it's advantages and limitations like any other product and doesn't exactly "shine" when used in a way that's outside its capabilities. MDF can not tolerate moisture at all (although they make "waterproof" versions that do fairly well.) I think the biggest reason why woodworkers don't like it is because it's really messy to work with. It makes a lot of fine dust when sawed or drilled, etc.

Stolen from Wood Magazine via Google Image Search

MDO is "real" plywood with a thin sheet of resin paper, plastic, or metal that is bonded to one or both sides to provide a smooth working face.

MDO is sometimes called signboard because sign painters (used to) use it for creating exterior, hand-painted signs, and so forth. It tends to handle moisture/water much better than MDF (which MDF is really, really, really bad at) so its good for exterior projects. It paints well, but its more expensive.

Both are well-suited to "shop" projects and so forth because neither is particularly "pretty". You wouldn't necessarily use either for show surfaces of "fine" furniture in most situations. But I usually cover the surfaces of my workbenches with MDF to take advantage of the density/weight.

There are also MDF-core plywood which feature an MDF core overlayed with veneer sheets which makes it appear like "regular" plywood. The advantage to this product is that it tends to be (and stay) flatter than other plywood while still taking stain, etc. like the "real stuff". It is heavier though.

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  • Nice overview Greg!
    – Graphus
    Oct 22 '21 at 7:51
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    Worth mentioning (unless I missed it) the the O in MDO is for “overlay,” referring to that resin/paper coating. There are also HDF and HDO, where the H is for “high” rather than medium. HDO is tough stuff — often used for concrete forms.
    – Caleb
    Oct 26 '21 at 18:37
  • Thanks, Graphus.
    – gnicko
    Oct 27 '21 at 23:02

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