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Thread: how to buy flat MDF

  1. #16
    Saw a pickup loaded with gravel last week that looked the same as car.

  2. #17
    The 4' X 6 ' sheet has about a 3/8'' cup, and several bricks on top did not flatten it. Clamping it to the web work of the torsion box might flatten it, but at a risk of twisting the grid beneath, and causing me a lot of stress during glue up.

    During a visit to another box store (not the same one), I put a 8 foot level on the top sheet of MDF, revealing a 3/8'' inch cup. Probably if I took that sheet home and put it on a flat surface, it would lay flat again. Still, I think I will visit a real lumber yard, not waning to have to struggle with warped wood. I choose to work with MDF because of its flatness, so it makes no sense to try to work with a crooked sheet.

  3. #18
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    Quote Originally Posted by Tom M King View Post
    We hand pick everything we use. The junk on top of the stack, whether board, plywood, or any other sheet, gets laid to the side, and we pick what we want. The junk gets put back on top of the stack. This goes for anything from framing lumber, on up. When the store gets a call for a delivery, it's the perfect opportunity to just grab the first pieces the help gets to off the top of the stack. I discovered this when I built my first house in 1973, so it's nothing unusual these days. Here, all sheet goods are stored flat until use.
    The one time I used enough lumber to justify delivery I was a bit scared of just what you described; but it was all good stuff. Maybe I got lucky.

  4. #19
    Okay, 3/8" cup. Grab a plant mister with water and spray the concave face so that it is evenly moist won't take much. Let it rest for 15 minutes and I'll bet you have a flat piece of MDF. Now glue up.

  5. #20
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    Quote Originally Posted by Chris Fournier View Post
    Okay, 3/8" cup. Grab a plant mister with water and spray the concave face so that it is evenly moist won't take much. Let it rest for 15 minutes and I'll bet you have a flat piece of MDF. Now glue up.
    what he said!
    When I die I hope my wife doesn't sell my tools for what I told her I paid for them.

  6. #21
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    Paul

    MDF comes in varying densities for any given sheet size. The stuff at the box stores is the "lowest" density, ergo it can sag quicker and faster, but it will all sag. Usually it can be fastened to a frame and the sag comes out. The higher density, thicker stuff, not so easily.
    Pat's comments are very relevant depending on the work and purpose of the MDF. If the sheet is sagged in the middle, it can make it harder to cut accurately if that's important.
    Given a significant enough sag, jigs, fixtures, and track saw rails will not, by themselves, be able to flatten the sheet. Inaccuracies and sloppy edges will result.
    I hate MDF with a passion, but it's a good product, so I would have to recommend that you pick and haul your own sheets if the flatness is important. I've yet to see a "Box Store" where the lumber and sheet goods were properly stored.
    "The first thing you need to know, will likely be the last thing you learn." (Unknown)

  7. #22
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    The cheap MDF from the box stores and also the construction supply places around here has taken on a distinctly cardboard like feel - it's not great to work with. Cheap usually Eastern plywood (even fancier grades like WBP and so called 'marine' - which actually meets no standard) has gone much the same way - it's often badly warped (and kinks badly after cutting when stresses are released too), is of variable thickness, has microscopically thin outer veneers, is full of stress, gaps, ply overlaps and weaknesses causing internal defects, has internal plies made from mush softwood that doesn't take machined detail very well, and as a result is basically floppy. Even birch ply from the wrong source these days can be very dodgy.

    I'm not in the US, but around here it seems with sheet materials to have become necessary to hand pick not just the actual sheets but also the brand and source (staying well away from the high volume construction supply outfits - there are a few that specialise in design/cabinet quality product), and be prepared to pay extra for better quality material. There's much better grades/higher spec of MDF about if you look, including water resistant and coloured varieties - and even anotehr steps up again to designer stuff: e.g. http://www.interlam-design.com/valchromat.html

    Over here no what sort of rubbish they sell it's always the best, and this that and the other. It often takes visiting and research to get to the bottom of what's on offer from a given source, because one lot of bland but generalised assurances are pretty much the same as the next...
    Last edited by ian maybury; 07-17-2014 at 6:52 PM.

  8. #23
    I'm with you Scott ,
    I have defininetly missed something .

    woodworking / carpentry 101 ,,, nothing is flat or straight ,, we make it that way .

    If you build a torsion box and a sheet of 3/4 MDF pulls it out of flat you have failed at building the frame , the MDF is just a skin .

    How big is this torsion box? And how thick are you making the frame ?
    Last edited by Thomas love; 07-18-2014 at 7:58 PM.

  9. #24
    Quote Originally Posted by Chris Fournier View Post
    Okay, 3/8" cup. Grab a plant mister with water and spray the concave face so that it is evenly moist won't take much. Let it rest for 15 minutes and I'll bet you have a flat piece of MDF. Now glue up.
    Great idea! If you want to add a nice lumpy texture.

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