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Discussion Forum

shrinking MDF Crown moulding?

johnzeitoun | Posted in General Discussion on December 16, 2003 06:04am

I installed about 5000 feet of crown moulding in a condominium last summer. It was MDF, primed and painted before I put it up.

I just got called back to the job because cracks of up to 1/4″ have been appearing between lengths of the stuff – at the scarf joints, not the corners. It would seem the lengths of moulding have shrunk in osme cases up to 1/4″ over 14′.

Has anyone ever seen this sort of behaviour from MDF before? I haven’t, though I’ve never used it as crown before.

It was put in in the summer, although it had, if it needed it, minimum two weeks to acclimate to the air conditioned interior of the building.

Also, what should I fill it with, assuming it may shrink more, or possibly expand again?

Thanks for any help!

 

John

 

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Replies

  1. noitall | Dec 16, 2003 07:04am | #1

    One problem that I had was using a water based glue on the scarf joint. It swelled the joint at first and then shrunk back once it dried out. It was a 5 1/2" crown so the joint was long and pronounced when the shrinkback happened. The install was in the winter though and only enough that once we refilled the surface of the joint with a light filler we had no more problems and all stayed put. I havn't seen any 1/4" gaps as you have though. Mine were only 1/16" or less.

    Scott T

  2. Piffin | Dec 16, 2003 07:20am | #2

    I have read documents reporting that thermal expansion in MDF can be as much as 3/16" in sixteen feet. I have never actually seen this much, but it seems like you now have.

    It sounds like the summer installation included leaving the material in a very warm place, such as under a sunny window, perhaps. Now that chilly dry winter climates are on us, the gaps are open.

    .

    Excellence is its own reward!

  3. seeyou | Dec 16, 2003 03:24pm | #3

    I'm seeing some slight opening of joints in my living room that I trimmed this summer. Nothing as dramatic as what you're seeing, but my longest pieces are 12'.

    1. RonnyK | Dec 16, 2003 06:23pm | #4

      I used 5 1/4" MDF crown in my living room this past summer.  My scarf joints in 12' lengths have parted about 1/16 - 3/32".  I am considering using a paintable highly flexible silicon caulk (GE has a new product called, I believe, Silicon X), priming, and then painting.  I had also considered using an epoxy wood filler-- the kind that you knead together to make a pliable "clay" that would then be pushed into the gap, smoothed with moistened finger and, if need be, sanded.  It seems the flexible silicon is the way to go, right?

      1. calvin | Dec 16, 2003 07:00pm | #5

        I would use urethane caulk to putty up that gap.  I think it will bond forever, and be easier to smooth out/mold to the contour.  Spit and/or thinner to make it smooth.  After it tacks up, you can push it in to mold it.  Takes paint well after curing.Remodeling Contractor just outside the Glass City.

        Quittin' Time

        1. User avater
          Mongo | Dec 16, 2003 07:39pm | #6

          Are you folks cutting the trim long and snapping it into place? Yes, even with scarf joints.

          The wost case I can think of it a long run I put in, about 22', with one scarf joint. No air conditioning in this house, baseboard heat, it's an old leaky house, so it suffers from seasonal swings in interior humidity/temp, etc.

          The joint is still tight as Day One. And yes, on Day One it was installed tight.<g>

          With scarfs, either glue them with a thin backer and maybe even a biscuit before installing. Or, install them one piece at a time. Put in Piece One, bow it, and nail it off only at the ends, leaving a slight bow in the middle. Then put in Piece Two in place, bow it, nail off the ends, then go back and nail off the middle sections of both pieces.

          Compresses them each a bit.

          Calvin gives good advice on caulk.

      2. johnzeitoun | Dec 18, 2003 09:37pm | #16

        I would use silicone. This is what we ended up doing, as a solid filler would probably crack or in any case, not allow for future expansion. The silicone would also allow for future shrinkage beyond where things are now.

  4. bruceb | Dec 16, 2003 10:38pm | #7

    We never use the scarf joint or Water based glue anymore.  We but joint it and put a biscuit in it. The first time someone mentioned it I thought he was a hacker. But, after i tried it, I was not at all displeased.

    when a carf joint opens up it does over a long area. A but joint opens ver a small area. Leave the material inside for a day or two before you put it up. When the time comes you can spring it into place and it will start with some pressure. That way any shrinkage will be minimal.

    Good luck

    1. RonnyK | Dec 16, 2003 10:45pm | #8

      What glue do you recommend with the biscuit joint?

      Edited 12/16/2003 2:45:51 PM ET by RKatcher

      1. PhillGiles | Dec 16, 2003 10:56pm | #9

        I'm with Robert on this. I use a butt joint with a biscuit and Weldbond (I know it cures slowly, but we make up all the long pieces in one pass and then install them later - we use Super T if we need to hurry, but it doesn't lock the biscuits in as tight). I try to plan it out so the long pieces have the square cuts at the ends (i.e. it's the pieces that tee into it that are coped), and that gives more latitude to hide a little shrinkage. I bow them when the humidity is high and just put them snug when it's low..

        Phill Giles

        The Unionville Woodwright

        Unionville, Ontario

  5. Taylorsdad | Dec 17, 2003 04:28am | #10

    Did you put in nailers first to attach the crown to, or just nail to the studs?  Also, what gauge nails did you use?

    1. noitall | Dec 17, 2003 06:36am | #11

      I use my 18 for everything inside and my 16 for everything outside. With MDF I find the 16s just mushroom up to high around the nail and take too much sanding to make it smooth. I have always found the 18s hold fine for all interior trim as long as you don't oversink the heads.

      I only use a backer to nail to if my crown is over 5 1/2" or a stain grade without caulk being used. With a 2" nail I am still hitting the plates and I can even nail my copes together if needed. I don't send too much crown up that doesn't have at least one coat of paint either.

      Scott T

  6. ccal | Dec 17, 2003 09:12am | #12

    I dont scarf my paintgrade crown anymore either. I butt it with a small backer behind the joint and if its too small for a biscuit use the instant super glue from fastcap. I am not a huge fan of mdf crown but have not had any swelling or shrinkage problems. I spring it in very tight like Mongo describes.

    1. PhillGiles | Dec 17, 2003 09:29am | #13

      What are using for backer ? My current mentor uses 3/16" Finish plywood and epoxy and it sure looks pretty good..

      Phill Giles

      The Unionville Woodwright

      Unionville, Ontario

      1. ccal | Dec 17, 2003 09:40am | #14

        I use strips of 1/4 inch ply. If I have time and the lengths are not so long as to be unworkable, I have been preassembling the joints on the ground before I hang it. This makes it easier to get a smooth joint than working from the ladder.

  7. shedhappens | Dec 17, 2003 05:50pm | #15

    I feel for you -- it sure sucks when you do everything right and you still  have problems.  I have a suggestion to fill the gap.  I've found that 'great stuff' foam spray sticks like poo to a blanket, has a little give and also can be shaped somewhat.  (hold a hack saw blade at a right angle  to the surface and saw lengthwise back and  forth lightly.)    since you're going to put some spackle over the foam anyway, any marks you're making will disappear.  I don't like these cheezy solutions, but it seems worth a try.  you might be able to make latex caulk work instead of spackle, though it is pretty hard to tool perfectly.

    and since we're on the subject -- you know when you use that foam for large gaps?  well, don't try to trim it with a knife blade -- use a notched trowel -- works great and keeps your knuckles out of the way.

    1. johnzeitoun | Dec 18, 2003 09:45pm | #17

      I thought about using Great Stuff, but opted against. Went with silicone caulk, and it worked OK, not great. In the areas where the light really shines all around the hallways, it will show, but hopefully it won't show too much.

      Good tip on trimming the expanding foam, I will try it in the future.

    2. AdamB | Dec 18, 2003 09:46pm | #18

      For triming the Great Stuff use one of those japanese pull saws.... the ones with the really flexable blade

      the blade will almost bend in a circle.  impressive and a couple of strokes will make a very smooth cut .  you still need to fill with something at that point... maybe some caulk would help?Christmas is coming..... should I buy the wife that new tablesaw ....hmmmm

      1. shedhappens | Dec 19, 2003 04:00pm | #19

        I agree with you about the pull saw.  I do have an old one that I don't mind using for stuff like that -- I just hate to disrespect the tool. -- no doubt you've also tried it for trimming shims instead of busting your knuckles  with a knife.

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