Thread: Help please - Jarrah table top bowing after glue-up

In one form or another it is moisture related. Moisture exchange between the air and the wood is happening unevenly on the two faces, or the wood isn't as dry as it should be and is cupping as it dries. Whenever you glue up a top, try and get it sanded and a coat of polish on it as soon as possible, this will help next time.

What amount of cupping are we talking about and how thick is the top? Depending on the design of your table, a fair amount can be pulled out when you screw the top down with buttons or whatever.

There are more radical solutions, like placing the top concave side down on grass on a sunny day and leaving it for a while checking it periodically. (The grass will give off moisture which will be absorbed by the concave face, which will expand, reducing or removing, or if you leave it too long, reversing the cup.) These methods can get you into more trouble though, so if it's a relatively small amount of cup then I'd probably advise against it. If you do try it, again, get it sanded and polished ASAP.

EDIT: When you say bow, do you mean it is curved along the grain (bow) or across the grain (cup)? The above post is assuming cupping.

Hi Scissors - Merry Christmas!!

Many thanks for your reply - particularly today. First of all, it was indeed cupping with dry Jarrah boards 30mm thick (finished size).

You must be right about the moisture and differences between the surfaces - I went in to the workshop to inspect my disappointing effort and found that the table top was flat again!!

I've never seen that before but then again I'm not particularly experienced either. It must have been taking the table top from a stable and cool environment to the outdoors on a sunny day. It would appear that the top reacted quickly to the heat ( direct sunlight was on it) and must have given off moisture rapidly. I must admit I never saw that coming and it happened very rapidly.

Your advice about quickly finishing the top is duly noted. I have covered the work today and will be applying the finish tomorrow.

Thanks again.

A woodwork class I took once suggested that if you were placing several panels together to form the top, you should alternate the direction of the grain and the direction of the heartwood. So that side on the grain looks like it is curving up in one piece,down in the next and up again in the third. It works for me. Mind you, if the top is all one piece then that blows that thought out of the water ...

Hope you have a great remainder of Christmas Day ...

You're more than welcome. It's amazing how fast wood will cup, particularly in strong climatic conditions, like our Australian summer. It's great that your top corrected itself. It's a rare but pleasant surprise when that happens!

The alternating grain thing will help for seasonal changes, but won't help for unequal moisture exchange on the faces. For a standard table top, where it is buttoned to the rails I don't think it's necessary, but if it's unrestrained then it's certainly a good idea.

I checked and I did not alternate the grain direction as has been suggested - maybe I will look at it next time but the the top will be buttoned down so hopefully all will be OK.

I had another episode of cupping - very slight this time, which again mostly corrected itself. I quickly sealed the whole top with the Danish oil finish I had chosen for it and left it for 24Hours.

That certainly stabilized everything and I have now finished sanding it flat and recoated with Danish Oil - which I would think should now keep everything under control.

I am still amazed by the speed of the cupping with perfectly dry timber. Thanks guys for your most helpful comments.

I've got a coffee table made out of spotty. I can tell how dry the air is by one of the corners. There is no gap at the moment (due to loads of rain in sydney), but around august this was out to a couple of mm. No problems with the quarter sawn timber - it is only the central grain that I have problems with.

The table is danish oiled then EEE'd, so whilst this may reduce the expansion / contraction it may not eliminate it.

Cheers,

Tom

Originally Posted by TomH

I've got a coffee table made out of spotty. I can tell how dry the air is by one of the corners. There is no gap at the moment (due to loads of rain in sydney), but around august this was out to a couple of mm. No problems with the quarter sawn timber - it is only the central grain that I have problems with.

The table is danish oiled then EEE'd, so whilst this may reduce the expansion / contraction it may not eliminate it.

Cheers,

Tom

Maybe mine is destined to follow a similar pattern. This is the first time I've used Danish Oil. I was able to get a wonderful finish from it (using Rustins with buffing) but I have since learned of its limitations as a wood protector.

Does yours stain much with water, coffee etc?

Finish sanding it quick smart and move it inside with the whole thing clamped between jointed boards to keep it flat. Leave it inside a few weeks to aclimitise to where it is going to live before you apply the final finish to it and then take it back inside until you are ready to mount it on it's legs.

This type of cupping to glued up boards is pretty common with the types of temperatures we have been having. I try not to leave glued up tops in the shed for any extended periods unless they are clamped flat due to the extreme swings in temperature that sheds tend to experience.

TP1,

It has been 6 months since you Danish oiled your table. Are you still happy with the result? Has bowing been solved by fixing the table to the base/oil finish? How does it react to big swings in humidity?- Last- Have you also oiled underneath the Table - for similar moisture diffusion rate top and bottom? Cheers.

Originally Posted by Tete de Bois

TP1,

It has been 6 months since you Danish oiled your table. Are you still happy with the result? Has bowing been solved by fixing the table to the base/oil finish? How does it react to big swings in humidity?- Last- Have you also oiled underneath the Table - for similar moisture diffusion rate top and bottom? Cheers.

It hasn't been 6 months yet and I haven't finally finished the table. This will be done this week.

I have oiled all sides and have brought the top inside and it seems to have stabilized. I think my problem stemmed from the fact that I did not apply the finish quickly enough after gluing/sanding and the weather extremes were able to have an impact. In future, I will also clamp across both the vertical and horizontal axis (even though the top was flat following the glue up)