Thread: Cupping after oiling

Originally Posted by chode

Or that the tops have had a little dose of direct sun?

Yup. The sunned top side is the concave surface right?

To avoid for the future, stand them on edge out of the rain, wind, snow, hail and sun. Not near a heater or air vent. somewhere that there is not epic air movement. Or the harder way is to sticker the boards with 1" gaps top, bottom, left, right. But if you have fresh finish on the boards, the stickers often stick to the curing finish and leave marks.

To fix these boards. Just sun the convex side for a bit and be sure to say "abracadabra!"

Thanks Kuffy,

Sun was my initial suspect but it appeared to be the worst on two boards in different room positions (in and out of sun) so that made me think it could be something else. However, I just remembered that I actually moved one of these boards the other day to get it out of the sun.

I'll flip them over and put them back where they were in the sun and hope for the best.

Cheers

Jarrod

The other thing which is a common mistake is to leave boards exposed to the atmosphere on 5 faces only. They leave them face up directly on the bench. Which allows the face to absorb or lose moisture while the underside is protected. Think of a sandwich left on the dining table on a plate for a few hours. The top side drys out while the underside remains soft and juicy. The answer, elevate the boards from the surface using sticks.

Yes I'm all over that one thanks! Due to the dust in the workshop I've been oiling in the back room on the floor but elevated on sticks. The sun only sits on them for a short amount of time and it's only been a few days so I thought I could get away with it. Guess not.

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If there is even the slightest bit of moisture in the material. that could swell the underside of the timber, making it cup upwards. A bit of sun for even a short time would probably compound the problem.

Turning the boards over might fix it if there is any dampness in the material.

Common fix for this problem is to put cupped side of timber face down on the grass and the moisture would reverse the cupping.

Hope this is of some help.

Cheers - Neil

Thanks for the tip Neil.

If if I gave no luck with them just sitting in the sun I'll put them on the grass. It's only really minor but enough to play havoc with my shelf separators.

Cheers,

Did you put the same amount of coats on top and bottom?
Having unequal coats can play a part in cupping simply from one side being more sealed than the other.

Cupping is caused by moisture imbalance on the two sides of the board. The tissue on the less-sealed underside has either absorbed moisture and expanded or tissue on the well-sealed upper surface has shrunk because the sun has driven out moisture from it. That is why, for most new work it is important to seal both sides of the board with an equal number of coats. (Historically this was not done with antique furniture because timber was well air-dried before use and only very stable timbers (like Australian cedar) were preferred. But with timber today it seems to be almost essential.

If you have to resort to the 'grass' technique I have found that:
1. Wait for a sunny day
2. Give the grass a light spray with the hose first
3. Place boards with cup to the grass (upside down U) (you are trying to get moisture into the tissue that has not expanded)
4. Wait until straight then quickly coat the topside in this situation (i.e. the original underside) with more oil to get equal coats top and bottom.

I have had mixed success with the technique. The boards always seem to flatten out beautifully in the sun, the question is whether they stay flat. Sometimes yes, sometimes no. It seems to depend on the inherent stresses in the wood and the timber itself.

What timber is it? Quarter sawn Tas Oak should straighten out OK, Radiata or Slash Pine cut backsawn from small trees maybe not.