sealing MDF, a revisit


Hello Group, I did a google search on this subject and most of the reasons were to lock in the vapors. I want to use a very heavy (1&1/2 inch thich) former computer lab desk for my router table. I have routed the cutout for the insert but I'm reluctant to set the insert in without treating the raw material. Do you think there is any chance the MDF could swell when polyurethane is applied? (Also, if that were a problem, I suppose I should avoid aqueous polyurethane.) My insert fits very nicely right now (minimal tolernces) but I'm concerned that it might become a problem if coated. Any comments? Thanks in advance,
Marc
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marc rosen wrote:

the thickness of paper is going to make the whole panel of MDF swell?? I don't THINK SO!
Dave
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thickness of paper is going to make the whole panel of MDF swell?? I don't THINK SO! I think the OP was asking whether sealant on the cutout section would swell the MDF enough to affect the fit of the insert. In response to that, I doubt that the amount of moisture in sealant would cause enough swelling to cause problems with your insert, but I know MDF does swell when wet, so I'd use a couple very thin layers of something oil-based that polymerizes to seal and protect from water. Of course if you have any scrap you could measure it with a caliper or micrometer and test a few different finishes - let us know how that turns out. Good luck, Andy
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marc rosen wrote:

you're probably fine. even if the edge does swell a little from water poly it'll stop as soon as it dries, and if the swelling is enough to affect accuracy it will be easy to sand it back to size.
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A coat of two of poly won't swell it, but they will reduce the tolerances. Are you sealing it for vapor? If so, that seems kind of anal to me.
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Hey All, No, I am not trying to seal in any chemical vapors. The entire sheet has some type of very hard and slippery material (melanminre? ) top and bootom. I want to seal the machined area to minimize material fragmentation due to use. The recess I machined was a perfect fit for my router lifter, and I anticipated that within several hundred hours of operation that the metal (anodized aluminum) to MDF junction would become sloppy from vibration if I did not seal the MDF. However, since I never worked with MDF before, I was curious if the small region that is sealed would swell, thus making my now perfect fit, not perfect. As I type this I am thinking that even the polyurethane may erode after hours of vibration so maybe I'll just go with the untreated recess and see when if it needs any attention in the future. By the way, the router lifter does have leveling and tensioning adjustments, but the recess is sized perfectly for height, with a minor amount of "slop" for lateral adjustments.
Marc
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marc rosen wrote:

the lift opening due to vibration in 4 years of usage. You MIGHT be overthinking this a bit, Marc. :)
dave
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David wrote:

Same here, and it's in a basement.
Barry
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