PuckJunkie

Poly finish causing warp?

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HI all -

About three years ago I built some basic shelves to fit under a raised bed. They fit great, but I was never happy with the finish, which I let the family do and was just acrylic paint. When we moved six month ago, we got rid of the raised bed and I took the opportunity to put a top coat on the shelves. I briefly sanded, wiped it with tack cloth, then put on three coats of a water-based polyurethane finish. It looked much better (although the entire episode convinced me I have a need to learn a lot more about finishing).

Yesterday I noticed that now the side of one bookshelf is bowed outward significantly. You can see the basic structure from the picture. The bookshelf is about 4' high and has a top, middle shelf, and bottom shelf that are all fixed; the left side is warped outward between the fixed points so that the floating shelves now barely engage the shelf pins. This is only on the front of the shelves, as the back is held together by 1/8" ply that is also apparently holding it solidly in place; from the back, the board is flat, and warps more as it gets to the front of the shelf. I hesitate to blame the poly, but those shelves sat entirely unchanged for two years before I put that top coat on.

Obviously the finish is on both sides, although I have read repeatedly that the idea you have to finish both sides of a piece of wood is an old wives' tale. What I don't know is what could cause something like this. I just want to try to prevent something like this in the future - this was a small project, but I have bigger things in mind down the road!

I also need to fix the issue, and the only fix I see is to convert those two floating shelves into fixed shelves and screw them in, either from the outside and try to hide the screw holes, or from the inside using pocket holes.

Thanks,

Puck

P.S. Go easy on my taste in books, those aren't all mine.

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Screws would work but would be the worst looking. Beings that it's already black you could do a dab of wood glue and some brad nails. If you don't have an air nailer hand pounded nails will work just make sure to pre-drill. If you had a clamp to hold things together while the glue dried it'd make things better. There is a chance that the water in the finish caused this but through acrylic paint seems odd.

Book collection isn't bad i saw Robin Hobb and a few Ender's books. I have the entire ender's game and ender's shadow series in first editions.

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35 minutes ago, BillyJack said:

"Old wives tale"?

It's beyond my expertise to make an assessment, but brief sidebars in woodworking magazines and the occasional online article like these all have made the claim that finishing both sides of a piece of wood is purely an aesthetic choice:

https://www.woodshopnews.com/columns-blogs/finishing-both-sides-is-warped-thinking

https://www.popularwoodworking.com/article/finish_both_sides_not_necessary/

I have no experience in the matter, but I've never seen a corresponding article claiming the opposite. Still, as I mentioned, this wood is finished with both acrylic and water-based poly on both sides, so it shouldn't be a factor here.

38 minutes ago, Chestnut said:

Screws would work but would be the worst looking. Beings that it's already black you could do a dab of wood glue and some brad nails. If you don't have an air nailer hand pounded nails will work just make sure to pre-drill. If you had a clamp to hold things together while the glue dried it'd make things better. There is a chance that the water in the finish caused this but through acrylic paint seems odd.

Book collection isn't bad i saw Robin Hobb and a few Ender's books. I have the entire ender's game and ender's shadow series in first editions.

I have a brad nailer, I hadn't considered that. I'd guess the brads would be pretty difficult to see against the black paint? Not sure about how much glue would accomplish - it would be the shelf's end grain being glued up against a finished surface. I haven't tried that before, but it doesn't sound like a recipe for success.

I'm really puzzled by the cause. It is a water-based finish, but this only happened on one board, and over the top of acrylic paint? 

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19 minutes ago, PuckJunkie said:

I have a brad nailer, I hadn't considered that. I'd guess the brads would be pretty difficult to see against the black paint? Not sure about how much glue would accomplish - it would be the shelf's end grain being glued up against a finished surface. I haven't tried that before, but it doesn't sound like a recipe for success.

It won't do a ton but it'll do more than brads alone. You could try CA glue.

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As there appears to be no connection of the two sides in the front, other than the top and bottom shelves, I’d be inclined to believe that the weight of the books is what makes the side to bow out. I would screw the shelves to the sides using pocket screws if you have it. If not, screw from the sides, recessing the screws and cut plugs (or dowels) to cover the screws, sand  and touchup with paint. 

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I don't believe that the bow has anything to do with surface coats, but the loading and changes in humidity with the move are certainly plausible explanations.  What wood was it made from?

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The unit is poplar, 1" x 12" from a local box store. The HVAC explanation made sense to me, but there's no HVAC vent nearby. It's sitting in the same room as an identical shelf I made at the same time, from the same wood, and only the one side of the one unit is doing this.

The two sides are connected not just at the top and bottom, but also by the fixed middle shelf; the top is sitting in a rabbet, the bottom shelf and middle shelf are in dados, and all three are glued and screwed. The back is ply and is glued and tacked into place as well. The bowing is occurring between the top and middle as well as the middle and bottom, only on the left side.

I don't understand how the loading could cause outward bowing. I suppose if there were sufficient pressure on top of each fixed shelf, it could cause buckling below, but there's basically no weight on top of the unit.

I'm going to try some pocket holes, I guess. I still would love to figure out what did this to keep it from happening in the future.

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22 hours ago, BillyJack said:

"Old wives tale"?

 +1, Wherever you read that, I would stop reading. Somethings are just facts, like  moisture absorption and 2+2 is four. ;)

There are plenty of amazing statements made about all sorts of things woodworking and not. The idea that a sealed and unsealed wood surfaces will react the same to changes in air moisture is a bit of a stretch.  However, it sounds like these units were sealed with paint and a later full coverage of poly should really enhance that.

Even lumber milled, stored and handled well will yield the occasional reactive piece.  We are never happy about it but, happier when it shows up before we finish the piece.  As a fix, if you don't mind the middle shelf being fixed, you could pocket hole screw it into both sides for a no-show fix.

 

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39 minutes ago, gee-dub said:

 +1, Wherever you read that, I would stop reading. Somethings are just facts, like  moisture absorption and 2+2 is four. ;)

There are plenty of amazing statements made about all sorts of things woodworking and not. The idea that a sealed and unsealed wood surfaces will react the same to changes in air moisture is a bit of a stretch.  However, it sounds like these units were sealed with paint and a later full coverage of poly should really enhance that...

I would add if you doubt this do a quick test on some scrap.

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I think recoating the wood introduced "some" moisture, together with books possibly forced in tight could cause outward force and bow, plus there's a lot of books which equal weight and pressure.

 

-Ace-

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The books aren't packed in to cause any outward pressure, I could easily take books in and out. I will grant that there are a lot of books - they are double-stacked two deep, just like every other bookshelf I own.

The pocket holes worked, more or less. The middle shelf was already fixed, but I put pocket holes in the ones above and below it. Another learning experience: since I cut the floating shelves just a tad short to make it easy to move them, screwing the sides flush to them actually made them deflect inward. It's barely visible, but when I used a straight edge it was clear. Less than 1/16". Live and learn, I guess.

Is it possible that this is just insufficiently dried wood from the box store? One of those "reactive pieces" that gee-dub mentioned. Most of the wood sat around in my shop for a while until I got around to finishing, but I had to go back and get more when I screwed up a cut. That last piece was cut and clamped up that day, then painted the next. Maybe the paint slowed down any additional drying and it just took this long to happen? Seems like a long while.

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FYI,  The books won't cuase nay aoutward pressure.  But if there is enought downward load on the thin board ti can start to buckle sideways.  Try pushing hard on the ends of a yeardstick.

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My comment about books being packed in was in reference to the post immediately before mine that said "books possibly forced in tight could cause outward force and bow." I understand the concept of buckling, but I doubt the bow at the topmost floating shelf is from downward force on the side piece, as the only load above that point is what's sitting on top of the bookcase.

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