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Thread: How to make invisible MDF joints?

  1. #1

    How to make invisible MDF joints?

    I sometimes work with .5" MDF panels I join together in 90 degree, glued butt joints. These get sanded, treated with sanding sealer, spray primed, and spray painted, producing a joint that, at least initially, is invisible. Occasionally, however, some of these joints reappear within a day or two as one side rises (or the other sinks) slightly, less than .005" but noticeable. What causes this? Is there any way to prevent it? Thanks.

  2. #2
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    If you are using regular PVA type wood glue the water content along with the fact they remain flexible is causing the problem. If it's an issue try epoxy or other non water based glue that won't swell the fibers.

  3. #3
    Thanks, John. Can you recommend any specific non water based glues?

  4. #4
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    My go to is West System epoxy either 5 minute or fast cure. If you are doing 90's in MDF you could also try a miter joint with a regular PVA. With that joint I do the 45's and use a masking tape with stretch to hold the joint together. You won't get a reveal line with the miter.

  5. #5
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    I built a vanity top out of butt glued MDF this Summer. I used TB III for the joints. I waited a couple of days after gluing it up to sand the joints flush and finish it. I first sprayed it with BIN shellac based primer, sanded it smooth, then another coat of primer, then 2 coats of WB paint. I was curious to see if the seams would telegraph through but, so far, they have not.

    John

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    If it is critical, always mitre the joint. It's the only way to be absolutely certain. Epoxy glue etc does work but you can't be sure it will all the time. Cheers

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    Would this be one of the places to use polyurethane glue? Not water based so shouldn't cause swelling/shrinkage. I've never tried it - not a fan of MDF but worth a shot?

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    Quote Originally Posted by Curt Harms View Post
    Would this be one of the places to use polyurethane glue? Not water based so shouldn't cause swelling/shrinkage. I've never tried it - not a fan of MDF but worth a shot?
    Typically you wet one side of the joint with water when using Gorilla glue. Maybe other polyurethane glues are different; I don't know.

    John

  9. #9
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    I'm not a fan of polyurethane glue for anything. I have found it works well for a period of time and then fails completely. About 10 years. I have a set of stools here now where every single joint failed at the same time. Not an easy job to decontaminate each joint and use proper glue. As I said above, it depends whether you want to take a chance or be certain. Mdf is a good material if used within its limitations. It's not a substitute for timber, just a complementary product. It is ideal for architectural paints. Cheers

  10. #10
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    Locked miter joint...
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  11. #11
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    Quote Originally Posted by John TenEyck View Post
    Typically you wet one side of the joint with water when using Gorilla glue. Maybe other polyurethane glues are different; I don't know.

    John
    I'm not sure about its use on sheet goods but for wood I suspect people overdo the moisture requirement, at least in the humid parts of the country. If it foams up a lot it must be really strong, right? Actually, the opposite is true from what I've read. There has to be enough moisture present to support the reaction but more is not better. Too much moisture/too much foaming = weak joint. The first couple times I used it I added moisture because a certain plaid clad woodworker on PBS said to. Then I read that the only areas that really need to add moisture are desert areas where EMC might be less than 6%. I don't use polyurethane glues much but feel it has its uses.

  12. #12
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    We use Gorilla ot Titebond polyurethane glue for our burnt tables. Less separation...We have pretty much went to Titebond..
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