Question MDF Sunken Joints

Discussion in 'DIY Speaker & Subwoofer Building' started by marvinwright1, Apr 13, 2016.

  1. marvinwright1

    marvinwright1
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    Hi,

    I've asked this question on a few other forums with little success so hoping someone here can help.

    A year or so ago I built myself a speaker enclosure from MDF. I glued and screwed the joints, the joints were filled with U-POL P38 filler. After the glue was set I removed the screws, drilled out and filled with dowels. Everything was sanded down and then I sealed the MDF with watered down wood glue. The finish I was after was a high gloss black, I'm no expert just a DIYer but I was pretty happy with the finish.
    Anyway to my question, about 5/6 months after painting the joints began to appear, these had sunk, and where the dowels had been placed, these began to looked raised.

    Below are a couple of pictures which show my issue.

    IMG_3370.JPG
    IMG_3371.JPG

    I'm now thinking of making a new box but I don't want to make the same mistakes.
    Instead of using screws then replacing with dowels I can use internal batters to construct the box but my main concern is regarding the join lines that appear.
    One one of the forum's I posted they said it was caused by the vibrations in the box but I'm not convinced by that, the box is totally sealed.
    Another and seems to be the consensus that the mdf holds moisture and can shrink/expand, another forum suggested that after sealing to use wood hardner.
    Surely everyone's builds are not suffering this so what would be the suggested approach to stop/minimise this?

    Any advice would be welcome.

    Thanks,

    Marv.
     
  2. mattkhan

    mattkhan
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    not an expert but hey it's a free interweb :)

    AIUI a key reason to use MDF is precisely because it doesn't expand/contract like real wood so whoever said that is surely smoking something

    if the box vibrates so much that it moves the dowels then it suggests the box construction is inadequate

    I've made 4 boxes so far with no screws in sight because I don't want to have to deal with the finishing side of things after using screws. I don't have a problem getting a box to hold together as it's the glue wot done it. I believe a brad nailer is the tool of choice to get the best of both worlds.

    AIUI avoiding seams in MDF with butt joints is expert level finishing, certainly above my pay grade. Consider using different joints and/or a different finish to cover up for you.
     
  3. marvinwright1

    marvinwright1
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    Maybe going for a piano black like finish was a bit ambitious for my first effort :)
    I'm still very happy with it though and it still sounds awesome in comparison to my previous BK XLS400 but using a better driver I'm hoping I can get even better bass.
    When you say a different kind of finish, you mean like using veneer or different paint or even something else?

    Marv.
     
  4. mattkhan

    mattkhan
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  5. marvinwright1

    marvinwright1
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  6. Ringnut

    Ringnut
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    Surely it looks like the dowel has retained the moisture and subsequently expanded?

    Another option might be to build the enclosures using screw and glue in the usual way and then bond another layer of 6mm mdf to the outside thereby avoiding unsightly screw holes? That's how Gordy did it with my DIY project.

    Dual LMS Ultra build
     
  7. jonone

    jonone
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    Both my cabs are gloss black, Gordy built them I got him to mitre and glue them so there are no external screws for exactly that reason, he then screwed and glued internal batterns.
     
  8. jonone

    jonone
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    [​IMG]
     
  9. marvinwright1

    marvinwright1
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    They look very nice too, are Gordy's boxed made ready to paint or did you prepare and then paint them?
    Also is the paint sprayed or by brush?

    Marv.
     
  10. jonone

    jonone
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    They come in raw mdf, mine are sprayed with car paint and primer, how you would spray a car.
    I basically got Gordy to build the boxes and then gave them to a body shop.
     
  11. marvinwright1

    marvinwright1
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    Yeah, I think thats the way to go to get the best finish but I love a bit of DIY, I guess for your average DIYer you never going to get that finish.
     
  12. jonone

    jonone
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    To do good mitre joints you need a saw that's bang on otherwise they flair and you will end up having to do lots of filling, same with spraying by the time you buy the paint, strainers, wet n dry, tack rags and spray equipment you might as well just give it to someone......
     

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