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MDF cabinet construction to minimize print through question.

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  • MDF cabinet construction to minimize print through question.

    Hi Gang,

    Most of what I've built to date has been out of Baltic birch. I think it looks good and like that it is quickly and very easily finished with shellac. I recently built a couple of speakers using MDF and like how easy it is to work with, but feel I'm going to need to veneer or paint it to get acceptable looks. I want to avoid seam print through and have had nice results with mitered baffle, side and back joins. What I'm wondering is what is least likely to print through, a top which is inset, or one which is glued on top of the vertical panels? It may be 6 of one half a dozen of the other, but I thought I'd throw this out to experienced builders.
    Also, how tough is it to miter the tops of the vertical panels and miter the top to eliminate all seams. My feeling is that the degree of precision needed to do this is beyond my skill and equipment level, but I've never tried it.

    Best,
    Jay

  • #2
    Hi Jay,

    That's an excellent question, and one that will likely get a few different responses to consider. Miter joints can be a good approach since in theory they eliminate the MDF end grain (if one can really call that "grain"). This face of the MDF is what tends to expand or contract the most, resulting in the print-through or "ghosting" that you're trying to avoid. I ran into this on my Swope HT tower build before I knew any better, and the veneer lines are still plain as day visible to me... but pretty much only to me As for miter joints and the skill involved, you are correct that they require plenty of patience and an accurate table saw setup to make a box that doesn't turn out lop-sided! That isn't to say it's impossible, or really all that difficult, just takes more setup time and patience to do it right.

    Back to strategies to avoid this ghosting... What I have seen on other posts regarding this topic is to build your cabinets, then give them time to grow/shrink/do their thing. This could be a few months if you have that kind of patience. Kind of like this: Let them sit for a month, sand them smooth, sit them another month, check, sand...sit, check, sand.... repeat until happy or bored to tears. This way any MDF movement is handled, and you can sand the minute shifts in position smooth again before going for a paint or veneer. If you decide to paint, I've seen plenty of folks apply "sizing" made of 50/50 wood glue and water to the entire cabinet. In Wolf's case, I think he suggests straight white elmer's glue! After this dries, you can sand it smooth again. The point here is to get the cabinet to soak up the glue mix and seal off any pores that would otherwise suck in all your paint and primer. Not only will your painting go quicker, you'll use far less of it to actually paint the cabinet instead of filling up all the little pores in the MDF!

    I hope this helped! I'll come up and say hi when I see you at InDIYana in April!
    Voxel Down Firing with Dayton SA70
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    • #3
      If your vertical miters are good enough, your top miters should be too.
      Just make test pieces with scraps until you have your saw adjusted well enough.

      Or do the Advent/DCM thing, and put a nice real board on top.

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      • #4
        I like the way devinkato minimizes telegraphing seams http://techtalk.parts-express.com/fo...by)-Build-Log=

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        • #5
          If you're looking for virtually perfect external surfaces I recommend covering the entire build with 1/8" HDF sheets. I've started using it that way and they create a smooth finish to paint or veneer. Yep, it's a lot more work but if you miter and seal the seams you're going to get a strong, smooth, and durable finish surface. Best part? The stuff is only $7 a sheet at the Big Box stores.

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          • #6
            For small speakers I like the 6 sided mitered box taped, glued and folded up. For 5 side mitered box I like a separate front baffle. I can use the round over to hide the joints or use hardwood baffle. For the 4 sided mitered box I'd do the overlapping front baffle with round over and an inset back panel.
            John H

            Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

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            • #7
              A blast from the past, "trench method" Dave Pellegrene. Basically routing a shallow trench on each buttseam and filling with epoxy. He had good long term success.

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              • #8
                A fool proof method for any wood construction is to epoxy Formica (or Wilsonart) laminate sheets to the surfaces. They will prevent any telegraphing of the seams or wood grain.

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                • #9
                  I've used Marty's approach suggestion with 1/8" HDF several times, and it seems to work well. It's basically a souped-up version of sacrificial veneer. It goes on fairly quickly, and it trims easily with a trimming router bit, just cut it 1/8" too big, and trim with the router. Even so, I still do what Keith referenced... machine smooth/flush after assembly, then let dry/cure for several weeks, months, then sand again. I've done this several times and then did the sacrificial veneer/1/8" HDF approach. No issues so far with any of it.

                  Yes, it's a bit more work, but I make a lot of stuff and I still manage to get this done on all of my speaker boxes that need it. It's only an extra day or two of work and it makes all the difference long term.

                  But to answer your question, yes, getting the whole box mitered (6 sides) isn't easy... it pretty much has to be perfect.... But it can be done, I've managed to do it a few times somehow. If you have a good table saw that is set up accurately for angles and square to the fence, it's doable. The 1/8" HDF solution is a good second choice for keeping seams hidden. The 1/8" HDF doesn't seem to expand/contract enough to show through.

                  I thought Dave Pellegrene mostly used Bondo to fill the seams, but maybe he used epoxy for some tests as well. Either way, I think some of the seams he did using the 'trench method' ended up cracking inside for some reason, though I could be wrong. To me, it was a LOT of work, but his results were excellent, though.

                  TomZ

                  I should add that a few times, I've sanded, waited, sanded, waited, etc a few times and still had the seams show through. MDF basically sucks.
                  Last edited by tomzarbo; 02-28-2018, 07:05 PM. Reason: Caveat about MDF
                  *Veneering curves, seams, using heat-lock iron on method *Trimming veneer & tips *Curved Sides glue-up video
                  *Part 2 *Gluing multiple curved laminations of HDF *Cello's Speaker Project Page

                  *Building the "Micro-B 2.1 Plate Amplifier -- Part 1 * Part 2 * Part 3 * Part 4 * * Part 5 'Review' * -- Assembly Instructions PDF

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                  • #10
                    My art teacher called it "defying the medium".

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