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Thread: Red Oak Tabletops are cracking

  1. #1

    Red Oak Tabletops are cracking

    Hello,

    I'm having issues with cracking with my red oak table tops. I'm importing raw/unfinished solid red oak table tops from Southeast Asia and staining and finishing them in the US. The tops are in plastic bags and the end grain is covered with sealer to prevent moisture loss. The moisture of the wood when I receive it is about 9-11%.

    Before finishing, I sand off the sealer from the ends and sand the top and apply an oil based stain. Then I apply a self sealing catalyzed conversion varnish. A few days later, I'm noticing cracks an most of the table tops that I have finished. The moisture content on these is now 7-9%. My shop is pretty dry, with a humidity of about 25-35% and there is no humidity control. I thought that the self sealing varnish was supposed to slow down the drying of my wood but it seems like it's drying as if no finish has been applied at all.

    What should I do to prevent these cracks? I'm thinking about switching to a catalyzed sealer instead of the conversion varnish or buying a humidifier but I'm not too confident either one will fix the problem. I'm also nervous that these cracks will get worse in a few weeks. I've attached pictures.

    Thanks!

    IMG_0412.jpgIMG_0413.jpg

  2. #2
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    About 9 - 11% down to 7 -9% shouldn't cause red oak to crack. However, if your shop is 25% then the EMC is only about 5.5%. I've had problems with cracking when the RH gets down below 30%, which is 6% MC for wood that had been dried to 6 - 8%. How are you measuring the MC? Pin meter? If so, I would get a pinless meter and check the tops, or a pin meter with long enough pins to reach the center of the wood and see what the MC is inside. I'm suspicious the wood was never dried down to 6 - 8%.

    What's the RH in your area of the country right now, and over the course of a year? Is your shop lower than what's typical? Hopefully it is, because if so the easiest solution might be to get a humidifier and raise the RH in your shop to around 40% = 7.5% mc.

    Also, I would stop finishing table tops, leave the factory sealer on them, and wait until the mc comes down to 7 - 8%. Do any of them crack? If so, then it's time to have a serious conversation with your supplier about what mc the lumber was kiln dried to before the table tops were manufactured. For most areas in the US you want 6 -8%, but if you are in the dessert SW then it might need to be lower.

    The slowest moisture transmission sealer I know of is shellac and that is what I would try after the OB stain.

    The more I think about this the more I'm suspicious about the MC of the inside of the tops. If they arrive sealed in plastic then the MC likely was never 6 -8%. Unless you live in the PNW that would be a problem most everywhere in the US.

    John

  3. #3
    Thanks for the answer John. I am using a pin meter that has a scanning depth of about 2 inches and the tops are 1.75" thick, so I'm pretty sure what I've measured is the humidity of the center of the wood. Our supplier has also sent us pictures of their moisture readings along the way so I would be surprised if their lumber had a very high mc.

    I have left some tops out with the factory sealer on them and they do not crack or really drop in mc too much. The RH in my area goes from 20%-80% but my humidity monitor shows that inside my warehouse it ranges from about 20%-40%. The more I read about it the more convinced I am that I need a humidifier regardless of whether that will fix this issue.

    Is it normal for a self sealing PU to allow so much moisture loss in just a few days? I wonder if the tops are losing moisture so quickly that could be causing the cracking. If they lost that moisture over the course of a month instead of a few days, they may shrink more evenly and not crack.

  4. #4
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    What MC readings has your supplier reported? If it's 9 - 11% then that's not low enough for most locations in the US. You haven't said where you are located, or where you are shipping your tables, but most locations in the US need wood that has been dried to 6 - 8%, some places lower, some a little higher. And with most homes now having AC the lower end is even more important than before.

    Like I said before, I had problems (with finished veneered panels) when the RH dropped below 30%. Your warehouse at 20 - 40% sounds like a real problem to me considering the MC those table tops are arriving at. What is the RH of your shop and finishing room?

    How long does your finishing process take? Two or three coats of catalyzed poly finish should have very low moisture transmission, so if you completed the finishing process in one to two days you may be able to prevent the tops from cracking.

    What type of sealer is the factory using? Sounds like good stuff. Maybe too good. Why do they apply it? They are already putting them in a sealed bag. If the wood was dry to 6 - 8% there would be no need for sealer to prevent moisture loss, only to prevent moisture gain.

    Is there any way you can finish the tops w/o removing the factory sealer? Just a thought.

    John

  5. #5
    Our supplier is reporting 8-10% MC for the lumber but I wouldn't be surprised if it is more like 9-11%. We are located in northern NJ and ship all over the US. They are telling us that 8-10% is pretty normal. Even if 6-8% is better, I wouldn't think that 8-10% would cause cracking within a few days.

    The RH of the shop and finishing room has ranged from 25%-35% recently so seems like that is definitely too low. I want to try to increase that with a humidifier to 50% and hopefully that will fix the cracking. I'm also thinking about switching to a stronger sealer and then a coat of catalyzed poly instead of two coats of catalyzed poly.

    Right now I am using a stain and then two coats of catalyzed poly. Although, after I apply the first coat, I sometimes let the tops sit for a few days or even a week. That second coat will probably seal it off better but I don't always get to apply the second coat within a day or two. I can't really finish the tops with the sealer on because those spots won't absorb the stain.

    I'm not totally sure what sealer the factory is using. They applied it on my request because we wanted to keep the moisture content from changing during transportation. The sealer is only on the ends of the table top though. How would putting the sealer on cause issues?

    Thanks,
    Greg

  6. #6
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    OK, northern NJ is pretty much the same as where I am in western NY, maybe even more humid. My shop runs 30% or less in the Winter, too, so I run a humidifier to keep it at 35% or higher. For my basement shop that's not a big deal but you might need an industrial solution if your shop is large. In any case, it should help a lot. But if you are shipping these tables all over the country then some of them are going to places where the EMC is 4% at certain times of the year. Wood should be dried for the locale where it's going to be used. Of course you can't specify 4% for some table tops, and 9% for others, but I think you will continue to have problems if your supplier is only drying them to 8 - 10%, or more likely 9 - 11%, or possibly even higher if you're measuring 9 - 11%. I think you need to lean on them to certify the tops are dried to 6 -8% and verify it when you receive them.

    I don't think the sealer is the root of the issue. It's just masking the underlying one. Unlike white oak, red oak is pretty forgiving stuff which says there is an issue with how the wood was dried.

    John


    My shop will run 60% or higher RH in the Summer, so I run a dehumidifier to keep it below 60%.

  7. #7
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    Greg, the cracks were most likely already in the wood when you received it and pre-existed when the end sealer was applied. The fact that they become visible is due to the slight MC% loss.

  8. #8
    I'm going to get a humidifier and see if that does the trick. I'm hoping that if they lose their moisture over the course of a week instead of overnight, the wood may shrink more evenly instead of all of the wood rushing through the path of least resistance and opening up cracks. Let's see if that works.

  9. #9
    Does anybody know how applying sealer to the edges of the table tops to prevent moisture loss might cause cracking? I'm wondering if that might be the issue. As John mentioned, maybe the sealer the supplier uses is too good but I'm not sure why that would be a problem.

  10. #10
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    I'd take a longer look at Scott Smith's post above. Drying a little more in your shop shouldn't cause end checking unless you are only letting the ends dry rapidly and not giving the tops time to equalize properly. You should be stacking them with stickers until they are as dry as they are going to get. I'd do this after removing whatever sealer the manufacturer is using.

    The tops are probably being made up using wood that was not dried properly which is causing the checks. While the MC is higher, these checks are closed tightly and are invisible. After drying more, they open up and reveal themselves. You would definitely not see them with a thick coat of anything on them.

    Are the cost savings significant enough to buy oak tops made in Asia? Oak is pretty easy to source here and those are pretty simple looking tops.

    Dan

  11. #11
    Quote Originally Posted by Scott T Smith View Post
    Greg, the cracks were most likely already in the wood when you received it and pre-existed when the end sealer was applied. The fact that they become visible is due to the slight MC% loss.
    I agree with Scott. I often work with Red Oak and I check the ends of the boards before I cut them to length and I often see checking. I get it fro a mill where the lumber has been dried to 7-8% ans surfaced on four sides. I take care to cut at a half an inch or more beyond any sign of checking. That practice has served me well over the years.

    By the way, having visited my daughter in Southern New Jersey on may occasions, I find it hard to believe that your shop could be at 25-35% you must work your A/C really hard. . I just checked the Rutgers weather site and there isn't anywhere in NJ that is below 88% humidity.
    Last edited by Lee Schierer; 05-29-2020 at 7:57 AM.
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  12. #12
    Thanks for the answers guys. Lee - The weather changes quickly in this area. The humidity was 25% ten days ago and now it is up to around 60% in the warehouse.

    It seems like raising the humidity fixed the end checking but I'm still seeing surface checks develop. They are smaller but still visible and only take a few hours to form. I have two ideas about what might be causing them. The cracks usually run down a single plank on the top.

    1. The sealer on the ends of the table top is very strong and when it is removed it is causing very rapid moisture loss. I'm not seeing how that would affect the surface so much if the sealer is on the ends.
    2. Some parts of the tops may be exposed to sunlight for a few hours and that might cause some cracking. I feel like oak is not going to be that sensitive to sunlight that quickly though.

    What do you guys think?

  13. #13
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    If you leave the tops in the sun for hours and they have a lot of moisture in them, then yes, you could be causing the checks. But I still bet they are there before shipping. In one of the photos you provided, it looks like some kind of thick sealant is embedded in the crack, leading me to think that it was there the whole time.

    Dan

  14. #14
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    All the info. points to a problem with the wood before it was shipped to you. Surface checks, in particular, are a sign of drying too fast in the initial stages. Maybe as has been suggested it would be better to find a local source. Regardless, I'd be strongly thinking about rejecting this whole shipment. Your reputation is going to take a serious hit if these tops crack after the customer gets them.

    John

  15. #15
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    Quote Originally Posted by Greg Libson View Post
    Does anybody know how applying sealer to the edges of the table tops to prevent moisture loss might cause cracking? I'm wondering if that might be the issue. As John mentioned, maybe the sealer the supplier uses is too good but I'm not sure why that would be a problem.
    Greg, it's not the issue.

    Wood starts shrinking once it gets below fiber saturation point (FSP), which is around 30% MC. Green, your oak was around 80% MC, give or take depending upon the species.

    Wood loses moisture more quickly from the ends of the boards than the faces/edges. For this reason end sealer is only applied to the ends. In order to be effective, end sealer must be applied within 3 days of the logs being bucked to length, and it's best applied immediately after a fresh cut. End sealer applied 2nd or 3'rd day will not stop all of the end checks. End checks are like cracks in glass - once they start they tend to keep growing. The key thing is to prevent them from starting in the first place.

    Flatsawn oak is going to shrink around 12% in width as it dries from FSP to 8%MC. If it does not have end sealer applied, the ends of the boards will shrink faster than the rest of the lumber due to the easier moisture loss from the ends of the wood cells. It is this delta that causes end checks. The ends of the board shrink faster than the body of the board, and so the ends have to crack due to the stresses.

    Proper stickering with oak can help to mitigate end checks, as they usually won't grow past the stickers. Thus, green lumber is properly stickered when the stickers are placed within an inch or so of the ends of the boards. The weight of the lumber above the sticker helps to clamp the boards in the stack so that the lumber does not move easily.

    Typically un-endsealed lumber will develop checks up to the first column of stickers, but not beyond. There are exceptions to this when excessive stresses are involved. For instance, log width black walnut boards and planks that have a wide sapwood band running down the perimeter frequently split deeply from the ends. The reason why is that the sapwood shrinks at a faster rate (but same percentage) than the heartwood, and the stresses placed on the board from the tension can cause deep splits - well past the stickers.

    Most commercial operators are working with green lumber that is 6" longer than the desired finished length (8'6" green length for an 8' finished board). After drying and processing the boards are end trimmed, which gets rid of most of the end checks developed while drying (if the lumber was properly stickered).

    If dry lumber goes through some cycles of being rewetted, then the multiple expansion / shrinkage cycles can result in face or end checks.

    But here is the thing - properly dried and end sealed lumber that does not have preexisting end checks usually will not develop them when drying from 10% to 8%.

    Flat sawn red oak lumber shrinks around .0016 per inch per 1% of MC reduction. That's one and a half thousands of an inch, and the entire board is shrinking.


    Bottom line - those checks most likely did not *start* in your shop.
    Last edited by Scott T Smith; 06-03-2020 at 5:12 PM.

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