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I had some spare MDF lying around, and didn't have much to do with it. We needed somewhere to shove our grill and gardening tools, so a couple of screws later I now have a reasonably unattractive MDF shed, built as a 3:4 triangle to fit under the stairs of the apartment above us.

I just realized that this thing may not hold up the best in weather. What's the proper sealing method -- polyurethane, epoxy, ... -- to keep MDF well-formed in the elements? This thing shouldn't be getting all that wet (since it's going to be under the neighbors' stairs, which have open backs) but it will be dripped on regularly. Also, I live in LA, so humidity isn't really a concern either.

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  • Clad the sides in treated Feather-edge boards and cover the top in roofing felt.

  • Raise it off the ground to prevent it from standing in water and soaking it up from underneath.

    I suggest getting a few concrete slabs and putting it on top of them, making sure that the edge of the shed (cladding) sticks out over the edge of slabs a little to prevent rain from getting on top of the slabs. You could also make a frame of treated timber to stand it on.

I agree that plywood would have been better, but I would have clad that in the same way too.

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Use an oil based paint, not latex. You have to coat the entire thing, including the bottom. This will help repel water.

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  • Guess I stopped checking for answers too early; it's sitting out there and holding up in this great LA rainy season, and it's taking all the water like a champ except for the bottoms which are soaking up water like champs (it's a champion shed). Fortunately, I raised the shed off the ground (a la @flamingpenguin) so the base is fine, it's just the bottoms of the exposed edges.
    – kyle
    Jan 2, 2011 at 7:10
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Honestly, your best bet may be to re-build the structure using an exterior grade plywood.

Whatever solution you come up with to "weatherize" the MDF will only be temporary. Soon enough, there will be some damage done to the protective coating that you apply, and then when the next storm comes by you will have a soggy pile of sawdust instead of a shed.

Do yourself a favor and go get some exterior grade plywood, and just use the MDF pieces as templates to mark the cuts on the new sheets. Should not take you very long, and will certainly pay off in the long run.

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Exterior primer, followed by exterior latex (water based) paint would be way better than leaving it alone, it might even improve its appearances.

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You could try painting it, probably with a latex paint, but in my experience there's nothing you can do to get MDF to hold up to moisture.

If it just has water running off it, it might last a couple years with paint, but if any part of it is in contact with the ground where it could sit in a puddle, it will start to soften up and fall apart in short order.

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