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  1. #121

    Default Re: Best methods for finishing MDF

    Quote Originally Posted by dlneubec View Post
    That's the method I've been thinking of doing on my current project, where paint is used. A backed veneer should hide the joints effectively, I would think.
    On my son's tritrix, I used 3/4" red oak ply, and inset the bottom of the front panel and covered with a 1/4" solid red oak panel. So there are no plywood seams visible.

    DP

  2. #122
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    Default Re: Best methods for finishing MDF

    Would that be the post from mgrabow "Check Out This DIY Build".
    Yes.

    Joe.

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    Default Re: Best methods for finishing MDF

    Heh, that look on his face says, "My stereo is better than any of my friends stereos, and I know it." He's probably right. I wish I had speakers that nice when I was his age. Hopefully he'll appreciate them, and maybe want to build his own once he gets a job and can afford parts. Ahh, the American dream, father and son, building audiophile speakers together, side by side.

  4. #124
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    Default Re: Best methods for finishing MDF

    Quote Originally Posted by dlneubec View Post
    That's the method I've been thinking of doing on my current project, where paint is used. A backed veneer should hide the joints effectively, I would think.
    Dan,
    FWIW, With the tests I have been doing I am coming to the conclusion that the moisture that is causing the seams to show is from the finishing process itself. Every one of the seams that I have had show up on the panels shows that the piece b u t t ing into the other is expanding from the finish applied then shrinking so the end grain is sticking out past. Basically the moisture expands the MDF in its thickness then shrinks again when it disipates. Hope that made sense. The process seams to take a few days. Basically the time it takes for the moisture to dissipate from the MDF. My suggestion, if you are going to try the veneer method. I would seal the MDF, with what ever you chose. The glue you are going to use to put on the veneer would be fine. Then let it sit for about 72 hours for the moisture to dissipate. If the seam shows up fill it then reapply the glue and wait to see if that takes care of it. Then apply your veneer.
    Dave

  5. #125

    Default Re: Best methods for finishing MDF

    Here's my favorite things about this thread: I thought it was just me! I like painting my speakers and the damn seams keep coming out. I particularly just like to use regular semi-gloss wall paint, to match the trim in the room they're going in (usually white) and I roll it on so it's textures like the wall paint. I use a pretty good quality oil based primer underneath it, and oil paint over it, and I put on more coats than I care to, and they still come out. I build very tight cabinets with invisible glue lines all around, and sand them perfectly to 600, and the seams still come out. I've tried various sealing methods and the seams still come out.

    The one set I have painted where the seams don't show is the one I used textured paint on.

    Great threat and I hope there's an answer. I build them to listen to, not to look at, but I'm a perfectionist, and it when imperfections develop despite my best efforts, it's frustrating.

  6. #126
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    Default Re: Best methods for finishing MDF

    Quote Originally Posted by mslatter View Post
    Here's my favorite things about this thread: I thought it was just me! I like painting my speakers and the damn seams keep coming out. I particularly just like to use regular semi-gloss wall paint, to match the trim in the room they're going in (usually white) and I roll it on so it's textures like the wall paint. I use a pretty good quality oil based primer underneath it, and oil paint over it, and I put on more coats than I care to, and they still come out. I build very tight cabinets with invisible glue lines all around, and sand them perfectly to 600, and the seams still come out. I've tried various sealing methods and the seams still come out.

    The one set I have painted where the seams don't show is the one I used textured paint on.

    Great threat and I hope there's an answer. I build them to listen to, not to look at, but I'm a perfectionist, and it when imperfections develop despite my best efforts, it's frustrating.
    So far the seams that are trenched out over a 1/16" deep and filled with either auto body putty or polyester finishing putty are holding up good. The finishing putty is much easier to sand and doesn't clog the paper at all. Doesn't really seem to matter what sealer was used. Even the ones with out sealer with just primer and paint are holding up good.
    Dave

  7. #127
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    Default Re: Best methods for finishing MDF

    Finishes will do that, too.
    Especially those with alcohol in them, it absorbs atmospheric moisture and introduces it to the substrate.
    These effects a rapid and short term. Your advise to wait before sanding is spot on.

    Wait for the other humidity effects to show.
    With the finished samples, your shower test barely had enough time to allow them to react due to all the finish material. The finish was doing its job keeping out moisture.

    Keep them in a dry location now and observe over the course of a year.
    My hopes are high! Nice job!!!



    Quote Originally Posted by davepellegrene View Post
    Dan,
    FWIW, With the tests I have been doing I am coming to the conclusion that the moisture that is causing the seams to show is from the finishing process itself. Every one of the seams that I have had show up on the panels shows that the piece b u t t ing into the other is expanding from the finish applied then shrinking so the end grain is sticking out past. Basically the moisture expands the MDF in its thickness then shrinks again when it disipates. Hope that made sense. The process seams to take a few days. Basically the time it takes for the moisture to dissipate from the MDF. My suggestion, if you are going to try the veneer method. I would seal the MDF, with what ever you chose. The glue you are going to use to put on the veneer would be fine. Then let it sit for about 72 hours for the moisture to dissipate. If the seam shows up fill it then reapply the glue and wait to see if that takes care of it. Then apply your veneer.
    Dave
    ~99%
    Make me an angel that flies from Montgomery
    Make me a poster of an old rodeo
    Just give me one thing that I can hold on to
    To believe in this livin' is just a hard way to go


  8. #128

    Default Re: Best methods for finishing MDF

    Quote Originally Posted by Jim Holtz View Post
    I've tried a number of different methods of sealing the seams and the only one I've found that is permanent is 1/8" MDF. I cap the ends/surfaces with all of the seams using 1/8" MDF and then you just have a 1/8" seam that is on an edge to worry about.

    BTW, bondo is the best solution other than 1/8" MDF I've found for seams.

    Jim
    I haven't seen 1/8" mdf around here, but tempered hardboard is common.
    A couple passes through a thickness planer can get it down to 1/16", and the tempered surface is practically ready for painting...doesn't scratch as easily as mdf.

    Ease over the edge with a sanding block and you'll never know it's there.

  9. #129
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    Default Re: Best methods for finishing MDF

    Quote Originally Posted by pecker View Post
    I haven't seen 1/8" mdf around here, but tempered hardboard is common.
    Hi Pecker,

    My son used to work at a store specializing in hardwoods that also carried all thicknesses of MDF for their commercial customers. 1/8" is only about $8 for a 4'x8' sheet and is much easier to hide the seams with than the endless sanding and hoping the seams won't show through process.

    good stuff!

    Jim

  10. #130
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    Default My method for finishing MDF

    After multiple iterations of sand and try a new method this has worked the best for me. Best of luck this can be a maddening event to find the right process.

    1) Hand Sand any machined edge(like 3/4" roundover or end of **** jouint to 400grit
    2) Skim coat with Auto Body Finishing Compound and allow to dry fully
    3) Sand to get rid of all small pock marks down to 400grit
    4) Wipe down with tack cloth and then acetone until the paper towels aren't pink anymore(usually the finishing compound is pink)
    5) Immediately shoot with Bulls Eye 1-2-3 Primer Sealer Stain-Killer
    6) Allow to dry overnight
    7) Sand to 400 grit(the primer is very thick, but go easy)
    8) Wipe down with tack cloth then acetone until no more debris comes onto the paper towel(I use the blue ones from autozone so I can tell when I am done because the primer is white)
    9) Immediately shoot Appliance Hard Enamel Paint(this stuff is semi-self leveling and harder for us DIY guys to screw up, plus it is really hard and durable when dry)

  11. #131
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    Default Re: Best methods for finishing MDF

    I guess I should have named this thread" Best Method for Hiding the Seam in MDF". That is what I am trying to accomplish. From what I have learned there are many ways to seal and finish MDF. From all the methods I have tested they all work just none, except for the trench method, will hide the seam. I should say so far. Keep your fingers crossed. There are methods of doing the seam differently like Jim Holtz explains, with covering with 1/8". Also cutting the seam on a mitered angle. Both methods put it on the corner so it is less noticeable. In my experience it will still show just much harder to pic out. Both methods are probably less work then the trench method. Covering the seam with veneer may work as well, just seams like quite a bit of work and then you have the wood grain to fill, if you don't want it to show.
    All in all the biggest thing I find in any of the methods is to allow plenty of time for the moisture content to dissipate from the MDF during the finish process. I would say 72 hours or more at 70 degrees. I would also add it would be a good idea to sand after 24 hours. This will allow the finish to breath better to allow the moisture out. Another thing I will mention is the grit you chose to sand with. If you try to sand the finish before it is completely dry you are scratching the surface and this could have to do with why a lot of people thing it is necessary to sand with such fin paper. Allowing a lot of dry time between coats will make it sand easier, powder up, and will also help eliminate the finish from shrinking over the next 30 days from all the trapped moisture. I think by allowing more time for the moisture to dissipate will alleviate imperfections from eventually telegraphing through the finish. I learned in doing automotive work if you rushed the repair process a month or so later you would see waves in the finish that were not there to start with.
    I will start another thread on my results of the samples I have been working on and rename it so its about the seam and not methods of applying the finish. I will also put a link to it here
    Thanks Dave

  12. #132
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    Default Re: Best methods for finishing MDF

    Dave,

    Thanks again for the work you are doing. When you get a chance please consider constructing a step by step instruction on what you think is the best proceedure to minimize the effect of seams, based on what you have learned. That would include a methodology when you don't do the trench method. You have much of it in the post above, but it is hard to follow. For example, how long do you suggest waiting after glue up before sanding. Would you sand smooth, apply sealer, let set 72 hours and sand again. I'm sure you see what I'm getting at.

    Here's a construction thought. Can we not get the same benefits as the 1/8" mdf or hdf surface by using deep rabbet joints, say a 5/8" depth, leaving 1/8" of material left from the corner? Better yet might be doing a "double" rabbet joint (see below), where both pieces are rabbeted and the legs of the rabbet leave only 1/8" of material for the overlap in both directions. That way you have a seam 1/8" from the corner and you have only 1/8" of material to swell or shrink. With less material to enable shrinking and swelling, might that not reduce the effect? Especially since that 1/8" of material could very easily be permeated with glue and finish material, making it harder for the moisture content to change over time.
    Attached Thumbnails Attached Thumbnails Click image for larger version. 

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    Dan N.

  13. #133
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    Default Re: Best methods for finishing MDF

    Quote Originally Posted by davepellegrene View Post
    I guess I should have named this thread" Best Method for Hiding the Seam in MDF". That is what I am trying to accomplish.......
    All in all the biggest thing I find in any of the methods is to allow plenty of time for the moisture content to dissipate from the MDF during the finish process. I would say 72 hours or more at 70 degrees. I would also add it would be a good idea to sand after 24 hours. This will allow the finish to breath better to allow the moisture out.
    Dave - my daughter works in a coatings lab - she has often admonished me for being too impatient when finishing projects.

    The following info is what she refers to and is provided by manufactures that produce glue, paint or any "wet" based coating - ie water, oil or solvent based.
    - most will surface "dry" within a 8 to 12 hour period, however to "cure" to the manufactures target hardness and final long term moisture content typically takes up to 30days at a nominal 70 degrees and 40-50% relative humidity.

    It seems like your tests (which are every much appreciated, btw !) are pointing to general agreement with manufactures recommendations.

    I guess by nature we are all just too impatient in our enthusiasm with a new project.

  14. #134
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    Default Re: Best methods for finishing MDF

    Quote Originally Posted by dlneubec View Post
    Dave,

    Thanks again for the work you are doing. When you get a chance please consider constructing a step by step instruction on what you think is the best proceedure to minimize the effect of seams, based on what you have learned. That would include a methodology when you don't do the trench method. You have much of it in the post above, but it is hard to follow. For example, how long do you suggest waiting after glue up before sanding. Would you sand smooth, apply sealer, let set 72 hours and sand again. I'm sure you see what I'm getting at.
    Yeah, I didn't expect it to turn into a 7 page thread. I sure wouldn't want to read through the whole thing. I will write up each method i have tested then someone can choose what they feel will work for them or at least give them something to build from.

    Here's a construction thought. Can we not get the same benefits as the 1/8" mdf or hdf surface by using deep rabbet joints, say a 5/8" depth, leaving 1/8" of material left from the corner? Better yet might be doing a "double" rabbet joint (see below), where both pieces are rabbeted and the legs of the rabbet leave only 1/8" of material for the overlap in both directions. That way you have a seam 1/8" from the corner and you have only 1/8" of material to swell or shrink. With less material to enable shrinking and swelling, might that not reduce the effect? Especially since that 1/8" of material could very easily be permeated with glue and finish material, making it harder for the moisture content to change over time.
    I do think the rabbet joint would work better then gluing the 1/8" panel on the side, especially the larger the panel, as long as you don't damage the edge in the process. Coupled with dry time it would probably hid very well. Its like coke with lime said 30 days to completely dry.
    I came up with 72 hours between coats or after gluing up from installing hardwood flooring. They tell you to let it sit in the area to be installed for at least 72 hours before install to acclimate. This seems to be close to the time it takes the MDF to do 90 % of its shrinking after moisture is introduced.
    I think I am also going to do a sample with veneer to see how that hides the seam. I think the veneer would work well on the end seams of several layers glued together to make a thicker piece. Not sure if it will bridge the b**t seam. Maybe with enough dry time before installing.
    Dave

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    Default Re: Best methods for finishing MDF

    Dan,

    Rabbets that long and thin in mdf will break very easily. They'll also distort very easily during assembly.
    I'd expect a lot of spoilage during construction.
    A double rabbett is also a very demanding joint to cut. You have to maintain extreme precision in every dimension.

    Quote Originally Posted by dlneubec View Post
    Dave,

    ...

    Here's a construction thought. Can we not get the same benefits as the 1/8" mdf or hdf surface by using deep rabbet joints, say a 5/8" depth, leaving 1/8" of material left from the corner? Better yet might be doing a "double" rabbet joint (see below), where both pieces are rabbeted and the legs of the rabbet leave only 1/8" of material for the overlap in both directions. That way you have a seam 1/8" from the corner and you have only 1/8" of material to swell or shrink. With less material to enable shrinking and swelling, might that not reduce the effect? Especially since that 1/8" of material could very easily be permeated with glue and finish material, making it harder for the moisture content to change over time.
    ~99%
    Make me an angel that flies from Montgomery
    Make me a poster of an old rodeo
    Just give me one thing that I can hold on to
    To believe in this livin' is just a hard way to go


  16. #136
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    Default Re: Best methods for finishing MDF

    Quote Originally Posted by bobbarkto View Post
    Dan,

    Rabbets that long and thin in mdf will break very easily. They'll also distort very easily during assembly.
    I'd expect a lot of spoilage during construction.
    A double rabbett is also a very demanding joint to cut. You have to maintain extreme precision in every dimension.
    You may be right Bob. I haven't tried it, but I was comparing it to using a 1/8" mdf skin, which I would think would be as subject to breakage and more subjec to deformation, but certainly easier to cut.

    I agree that precision would be ideal, but since we are comparing it to hand sanding valleys in he mdf and then filling it with something, sanding that down, etc., I'm not convinced it is the harder or more time consuming road, even if the joints aren't perfect and need a little filling. It would come down to getting the initial setup right for the rabbet depths and once that is done on some scrap pieces to perfection, it should be just running the parts through, either with a dado bit or router on a table. It just doesn't look that hard to do to me. The one lapped piece could be cut a little larger, allowing for flush bit trimming after, if so desired.
    Dan N.

  17. #137
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    Default Re: Best methods for finishing MDF

    The 1/8 stuff has the benefit of being homogenous, with both dense outer layers in tact. An 1/8 thickness of that is exponentially stronger than the 1/8 of rabbett.

    When the skin is applied to an established base (an enclosure in this case) the skin would have plenty of support. It will conform somewhat to the base, but will bridge minor gaps without breaking, unlike a thin sliver of rabbetted mdf.

    Way back when I learned that mdf was not well suited for most traditional joinery. Once you cut through the face it loses a lot of its strength.
    Grooves, dados, rabbetts, some splines, t&g, etc are all pretty poor choices for mdf. They tend to produce a very weak joint rather than a stronger one.
    You can test this very easily. Make a rabbett like in your example and press gently on the end. Watch the piece flake off.
    Now rest an 1/8 mdf or hdf piece of same size on a bench with the same amount hanging over the edge. I didn't break with even with a good amount of pressure did it?

    For cutting that joint you have 4 reference or critical dimensions instead of two. If one length is off trimming will affect the overall size of the panel which can get you in trouble real quick.
    And the panels must be sized very accurately before you cut the joints, unlike a skin with those b(censored) joints or other b(censored) joinery methods.

    Doable for many, but for those without good saws and quite some experience it would be a real challenge.

    Anyway, either method is more work.
    I'm liking the trenches!
    We'll see come September.

    Quote Originally Posted by dlneubec View Post
    You may be right Bob. I haven't tried it, but I was comparing it to using a 1/8" mdf skin, which I would think would be as subject to breakage and more subjec to deformation, but certainly easier to cut.

    I agree that precision would be ideal, but since we are comparing it to hand sanding valleys in he mdf and then filling it with something, sanding that down, etc., I'm not convinced it is the harder or more time consuming road, even if the joints aren't perfect and need a little filling. It would come down to getting the initial setup right for the rabbet depths and once that is done on some scrap pieces to perfection, it should be just running the parts through, either with a dado bit or router on a table. It just doesn't look that hard to do to me. The one lapped piece could be cut a little larger, allowing for flush bit trimming after, if so desired.
    ~99%
    Make me an angel that flies from Montgomery
    Make me a poster of an old rodeo
    Just give me one thing that I can hold on to
    To believe in this livin' is just a hard way to go


  18. #138
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    Default Re: Best methods for finishing MDF

    Quote Originally Posted by bobbarkto View Post
    The 1/8 stuff has the benefit of being homogenous, with both dense outer layers in tact. An 1/8 thickness of that is exponentially stronger than the 1/8 of rabbett.

    When the skin is applied to an established base (an enclosure in this case) the skin would have plenty of support. It will conform somewhat to the base, but will bridge minor gaps without breaking, unlike a thin sliver of rabbetted mdf.

    Way back when I learned that mdf was not well suited for most traditional joinery. Once you cut through the face it loses a lot of its strength.
    Grooves, dados, rabbetts, some splines, t&g, etc are all pretty poor choices for mdf. They tend to produce a very weak joint rather than a stronger one.
    You can test this very easily. Make a rabbett like in your example and press gently on the end. Watch the piece flake off.
    Now rest an 1/8 mdf or hdf piece of same size on a bench with the same amount hanging over the edge. I didn't break with even with a good amount of pressure did it?

    For cutting that joint you have 4 reference or critical dimensions instead of two. If one length is off trimming will affect the overall size of the panel which can get you in trouble real quick.
    And the panels must be sized very accurately before you cut the joints, unlike a skin with those b(censored) joints or other b(censored) joinery methods.

    Doable for many, but for those without good saws and quite some experience it would be a real challenge.

    Anyway, either method is more work.
    I'm liking the trenches!
    We'll see come September.
    I think we will have to agree to disagree on this one, though I'll give your test a try. I think you might be overstating the weakness of the 1/8" lip of mdf. I have to admit that I don't recall using thicknesses smaller than 1/4" in the past, so perhaps a 1/8"x5/8" lip would be too weak. My guess is that if you have at least one hardened side intact, it will be sufficiently rigid to hold up unless you drop it on the floor, which would damage a 3/4" piece or 1/8" skin of any size anyway.

    In general, getting the dimensions correct is just not that hard, IMO. You simply have to get the depth of the dado blade or router bit setup correct once for each side of the joint and can usually run multiple pieces through with just a couple such setups, using scrap test pieces to ensure the setup is correct. If you run all your joints of the same dimension with a single setup, then there is no reason for them not to fit perfectly, assuming you did an adequate test run on a couple scrap pieces.

    I've used rabbets with most of the speaker builds and find them easier overall, when you consider accurately clamping and gluing them together after they are cut, and stronger than b##t joints. Either way you have to get at least one of the panels cut straight and to the right size since they can't all overlap. If you can cut a good 90 angle and get one panel cut the correct size, there is no reason you can't do them all accurately, unless you are in a hurry, are careless or simply make a mistake. Adding rabbets is no big deal if you have a decent router and router table or decent table saw and ideally a dado blade. IMO, they make the panels much easier to get aligned, striaght and square when assembled than trying to do so with b##t joints.

    I'm sure the skin concept can work well, but it is also requires extra time and steps, just like rabbet joints, requires a larger, probably heavier cabinet for the same internal volume and requires you to pick up and store both 3/4" material and 1/8" material. Ditto the trench concept, which requires extra hand sanding of the trench, sealing, filling, sanding, etc. Pick you poison, I guess. None come without costs in time and/or cash.

    Frankly, I'm holding out hope that using an inexpensive sheet of tight grained paper back veneer will be the easiest method overall and will adequately hide seams, but I'll have to test that as well.
    Dan N.

  19. #139
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    Default Re: Best methods for finishing MDF

    Quote Originally Posted by dlneubec View Post
    Frankly, I'm holding out hope that using an inexpensive sheet of tight grained paper back veneer will be the easiest method overall and will adequately hide seams, but I'll have to test that as well.
    I'm very pessimistic about that working. I've seen joints telegraph through maple veneer on two builds. Both have a glossy surface - one with polyurethane and the other finished with Danish oil - and the gloss amplifies the problem.

    The joints on both builds were invisible initially, but changing ambient conditions caused the joints to appear as slightly uneven areas on the surface. In recent projects, I'm now using two layers of paper-backed veneer combined with lower gloss finishes to minimize telegraphing of the joint.

    As a side note, here's something interesting and perhaps relevant...

    While Dave has been doing this experiment, I've been paralleling it by measuring a couple pieces of bare 4 x 6 x 3/4" MDF to determine changes in the size of the material. Since early December, I've seen a 0.010" change in the thickness of the MDF, and less of a change in the other dimensions. Drier weather causes shrinkage. I'm still taking measurements periodically, and we'll see what happens when damp spring conditions arrive.



    The finding so far verify what Dave has suspected - thickness changes much more than the other dimensions of MDF. I used two MDF specimens - one tan MDF, the other yellowish - to see if there is much difference between the two "species". There isn't.
    Bill Schneider
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    Default Re: Best methods for finishing MDF

    Quote Originally Posted by williamrschneider View Post
    I'm very pessimistic about that working. I've seen joints telegraph through maple veneer on two builds. Both have a glossy surface - one with polyurethane and the other finished with Danish oil - and the gloss amplifies the problem.

    The joints on both builds were invisible initially, but changing ambient conditions caused the joints to appear as slightly uneven areas on the surface. In recent projects, I'm now using two layers of paper-backed veneer combined with lower gloss finishes to minimize telegraphing of the joint.

    As a side note, here's something interesting and perhaps relevant...

    While Dave has been doing this experiment, I've been paralleling it by measuring a couple pieces of bare 4 x 6 x 3/4" MDF to determine changes in the size of the material. Since early December, I've seen a 0.010" change in the thickness of the MDF, and less of a change in the other dimensions. Drier weather causes shrinkage. I'm still taking measurements periodically, and we'll see what happens when damp spring conditions arrive.



    The finding so far verify what Dave has suspected - thickness changes much more than the other dimensions of MDF. I used two MDF specimens - one tan MDF, the other yellowish - to see if there is much difference between the two "species". There isn't.
    Bill,

    Based on your experience, I wonder if 20mil paper backed veneer would work. Was your experience with the typical 10mil?

    Still, I can't say I've noticed any seams telegraphing through the veneer on my unpainted projects, though they all have high a gloss poly finish. I don't even see any on my SoundRounds veneer, which have a very thin, unbacked veneer over mdf and they are nearing 2 years old now. Maybe it is just more evident on painted finishes. I'm starting to think I should just stay with a wood veneer finish rather than paint. Paint sounds like too much work, with a pretty unpredictable result!
    Dan N.

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Crossover Components

Cabinet Hardware & Speaker
Grill Cloth

Speaker Cabinets

Subwoofer System Kits

Speaker Kits

Speaker Repair Parts

Speaker Wire