Thread: Plywood endgrain table top bowing

What I do is lay down a sheet of any thing handy , MDF Masonite or chipboard, stack my work , my flat tops that I don't want shifting and cupping over night . And then cover with another sheet of any thing Handy.
This way , just the edges are exposed to the atmosphere , and nothing changes shape . In Melbourne this time of the year , as you know , we can have hot weather with dry northerly winds one minute and then a cool change comes through with rain , the relative humidity is all over the place . I did learn the hard way that not to protect your work from these swings means you may have to build the whole thing twice . It's a good way to learn how to concentrate on your work while swearing a lot.

I have not done that sort of work with plywood , but its important when gluing solid timber up for cabinet work like sides and tops of chests and cabinets and the tops of tables like extension tables , where the expanse can be wide and the thickness thin like a top 1.1 x 1.2 x.019.

Rob

Well, I've definitely learnt something new ( i'm pretty much just starting out ), so will now ensure I have things either sealed or pressed flat when i finish up for the day!

Agreed, with Melbourne and it's crazy weather. We had the Air con on the other night when it stayed around 27 all night! and now we've had the the heater on today! Crazy!

Hell what you are doing I would not attempt.

I've worked with plywood quite a lot and it can be tretcherous.

Firstly the moisture content of plywood can leave a lot to be desired......in general it is not dried anywhere near as well or consistently as we would expect good furniture grade solid timber or KD hardwood construction timber.

When drilling thru metal fixtures on a cold day....I have seen the moisture from the drilled ply condense on the fiting beside the hole, many times.

secondly some of it can have considerable stresses contained in it.

I have cut an 8 foot sheet up the middle and have it spring 6mm in the middle

I'd sugest you have a moisture content problem and ya piece is drying rapildy thru the end grain.

you have probaly put it in a position where one side has lost more water than the other.

spraying water on the short side probably wont do much for you, because the problem is full depth.

i've had a couple of pieces of ply in normal flat pieces do that on me......the only solution is time and dry weather.

I pushed my luck a couple of times and it just is not worth it.

I'd recommend that you put that piece in a way that is held more or less flat and equally ventilated both sides and wait for it to equalise before you go any further.

I've had pieces of ply that has been upset take 3 months before they came back flat and stable.

sorry
cheers

Good to know. And here I was thinking that Ply was more 'dimensionally stable' than normal wood!

Providing I can get it flattened ( and it has flattened nearly all the way over night by allowing ventilation underneath, whilst being weighted flat from the top ). Once it is flat, if I seal it - should I assume it will stay that way? i.e. once the sealer goes on, no moisture will go in/out, and thus it should keep it's shape? Providing the bowing was moisture related in the first place. This is also sealing top, bottom and all ends.

Anyway, I'm going to give it a go, and see how it comes out. Other than that, the project is coming along nicely

I should probably start taking some pics!

Ahh but ply IS way more dimensionally stable than plain wood....across its flat face.

But hell plain wood will often sing the whole "A space oddity" album with very little provocation.

You have put plywood in a situation that it is least stable in.

it is the very stability of ply in the plane that causes it to become unstable otherwise.

As far as sealing the whole thing and stopping moisture comming in and out.....unless you are drowning it in epoxy or at least two pack varnish you are dreaming.

Most common timber finishes are pretty porous and moisture goes thru them pretty well.....maybe a lot slower than bare wood.....but moisture most certainly does travel thru finish.

all it does is delay the disapointment.



At one time I built a lot of speaker boxes, equipment cases and cabinets out of ply.
I was frustrated by inaccuracies in dimension and squareness, till I started cutting all pieces slightly oversize and trimming all edges to size and square.

it is the internal stress that keeps ply dimensionally stable and strong......but those stresses redistribute every time you cut it.

Once it is cut to size and kept flat by the remaining structure a piece of ply will remain very stable.

This is why in general single flat pieces of ply make poor doors.

cheers