Technique to flatten warped table tops

I'm posting this here because it is a technique I learned from a restorer of antiques, but this example is a new piece of Australian cedar.

Some time ago I had this 1800 mm x 750 mm x 40 mm slab of Australian cedar flattened by someone with one of the YAS SlabMaster machines. It started at 50 mm and ended up being 40 mm. At first, the slab was beautifully flat, but a couple of years later, after being stored in a hot garage, it developed quite a side-side bow. I was going to use it for a bed head for my daughter, so I wasn't worried about the slight bow. I finished the convex side with Hard Shellac so it would resist wet hair etc. However, the cedar has been rejected - something more colourful and smaller is called for. I therefore decided to make it into a small table. But, as a table, it needs to be flat.

The flattening technique I learned about 30 years ago involved an antique Kauri Pine table that got too hot and the top curved up badly. The restorer told me to take the top off, wet the lawn in a sunny spot and drop the table on the wet lawn - polished surface upwards. It worked!

Here are some photos of the technique working on my cedar slab.

Attachment 386592 The slab on the wet grass
Attachment 386593The slab showing the bow
Attachment 386591 Slab flattened and being sealed on the back surface with shellac.

I have done a lot of straightening by wetting the concave side with a spray bottle of water and laying it wet side down on the workshop floor. Sometimes I can stand there and watch it flatten it is so quick and once it has flattened it seems to stabilise. I was building many tables out of finger jointed pine slabs and got pretty good at it.

Ive gotta put this in my bag of tricks

Thanks ... much appreciated

Regards


Rob

Water+sun does wonders. Lying it on or wrapping it in a damp towel also works. The problem I had was it worked too well, and had to be reversed a little...

Good tip and Thank you for sharing it.

The cupping of the board will happen again as the moisture content of the two sides stabilise or return to equilibrium. Sealing the raw side after straightening can help delay the recupping, for years sometimes, but unfortunately, it's inevitable.

Craig

It depends on how that cupping occurred and the subsequent use I suppose. In my case it was by stacking the boards flat one on top of the other and the top one always cupped caused by moisture trapped in the underside if left too long. Once straightened and both surfaces sealed with finish the boards haven't noticeably moved and they are all in service as tops for tables in a restaurant now. Just my experience which may help someone else and certainly not gospel on what may or may not happen. It used to amaze me how the application of water on a board could have such an immediate effect on some panels that were bent as much as 20mm in some cases.

I use water to flatten panels when I need a flat panel NOW. But if I have a few days up my sleeves, i just lay the panel concave side down on a sheet of plastic, or anything else which is suitable as a moisture barrier, and in direct sunlight. usually inside the house so i don't need to worry about rain or birds leaving unwanted deposits. Sometimes it may take a week until I am satisfied, but other times it happens while I eat lunch. Once it is flat again, I store it on a flat surface wrapped in plastic until I am ready to seal the board with whatever finish. I do this sort of thing most commonly for 6x1" boards which I have resawed in half, they seem to always want to severely cup..while resawing.

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Originally Posted by CAG

The cupping of the board will happen again as the moisture content of the two sides stabilise or return to equilibrium. Sealing the raw side after straightening can help delay the recupping, for years sometimes, but unfortunately, it's inevitable.

Craig

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I still have the Kauri Pine table top that I flattened with this technique over 20 years ago (it was 1993 to be exact). It is still flat. However, I am sure that some boards will be stubborn. It will depend on the individual situation to some extent.

Good to hear of some successful straightening. I guess it's a different story when the cupping is caused by internal stresses in the log when it is sawn as opposed to cupping due to moisture loss.

As an aside, I love the astonishment on my students' faces when they retrieve their work after I have told them to use water to raise any dings and dents in their projects; priceless!

Quote:

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Originally Posted by CAG

Good to hear of some successful straightening. I guess it's a different story when the cupping is caused by internal stresses in the log when it is sawn as opposed to cupping due to moisture loss.

As an aside, I love the astonishment on my students' faces when they retrieve their work after I have told them to use water to raise any dings and dents in their projects; priceless!

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Yes, indeed, sometimes just wetting the board will do it, other times I use a $12 KMart iron on steam setting and it removes most or all of the dings. The iron even has a water spray feature that gives a good wetting for larger indentations.

I think the wet lawn system works well for boards that were once flat. As you say, if the board is not naturally flat because it has stresses causing it to curve or as could happen if it was sawn wet and then curved as it dried then the technique may not work (probably would not work). I have primarily used it on antique tables that were flat and had been flat for 100 years but then became overheated or over-dry from some extreme exposure. The wet lawn technique returns them to their natural flatness.

Yes this works -but once straight . keep it that way by attaching to suitable fame (with allowance for movement) or sandwiched between straight bearers for storage.