Oak boards cupping and splitting

[neilr]

Hi folks,

I've made a few signs using oak and in one instance the board has cupped badly and there are numerous splits running the length of the board. Here's the spec etc:
Signboard is approx four months old! European oak - 25mm thk x 200mm wide and 900mm long (grain running along the 900mm). Lengthy name v-carved in script typestyle along its length but taking up perhaps only 1/6 of the surface area as strokes are relatively narrow. Max depth of v-carve 8mm. Board has been finished using three coats danish oil.

Owner didn't want holes drilled through the board as he wanted to mount it by using 'L' brackets from behind onto posts.

My own suspicions are that:

1. Carving has released surface tension allowing the board to cup - should it have bracing on the back?
2. Sign is located close to the sea approx 100M away - perhaps salt air has dried the face more that the back?
3. Use of (flimsy) 'L' brackets are too weak to hold it firmly against posts which would otherwise afford it some barcing.

I have no problem with making a new one and I am using the experience positivly to help me understand the underlying cause so that I can save it happening in the future. I can try out possible solutions but it could take a long time before I would be able to determine if they are working or not, so I'm hoping somebody here will have prior experience of this. Any suggestions and experiences much appreciated.

Cheers - Neil

 

[Phil Pascoe]

Make sure your oak is quarter sawn- it won't cup. I wouldn't have thought 3 coats of danish oil would do very much for exterior oak- oak is notoriously impenetrable- you might be better off going for yacht varnish or soaking the whole piece in a good preservative and leaving it bare( it'll go silver grey), but someone with more knowledge than me will come up with something better.

 

[andersonec]

With a board 250 wide it will cup unless it is braced well at the back and 900 would need a brace every 300mm or so, if it is out of one board, and I would guess it is, you will have cupping.

The first is the usual method of sawing a plank (end grain) and shows which way it will go, the second is quarter sawn but I doubt if you will find a piece 200 wide.
Danish oil outdoors is not very durable, best to go for something like Osmo UV protection oil, shame to let a piece of nice oak go grey.

Andy

 

[Jacob]

It'll go grey unless you oil it several times a year, then it'll go black. Paint?

 

[deserter]

Personally I'd be inclined to cut the 250mm board into 3 sections, turn the middle section back to front and rejoin using biscuits and exterior glue that way the grain should help stabilise the board.
Another idea is to add a frame around the sign which would also act as a brace.

 

[andersonec]

Jacob,
Oils such as Danish, Teak or Linseed will need going over fairly regularly but two coats of a UV protection oil will keep the wood from greying for four or more years depending on location. and if you are going to paint it then use a bit of treated pine or similar.

http://www.osmouk.com/images/2010%20UV- ... 0Extra.pdf

Andy

 

[Jacob]

Personally I'd be inclined to cut the 250mm board into 3 sections, turn the middle section back to front and rejoin using biscuits and exterior glue that way the grain should help stabilise the board.
Another idea is to add a frame around the sign which would also act as a brace.

Yebbut the spelling would be all wrong!

 

[Steve Blackdog]

Everything Andy says.

My 2p worth. Unless it's quarter sawn it will always move. Maybe finishing with Yacht Varnish would help, but it never ceases to amaze me what a bit of weather can do to timber - especially oak.

Is another answer to deal with the cupping) that 1" thick is simply too thin for an 8" wide board, especially one with lettering routed out to 1/3" depth. You could get away with it screwed down onto something, but I reckon I'd've preferred 1 1/2 inches thick for 8" wide (and still braced it).

Here's a point for the technically minded. Assuming there is no quarter sawn plank available. What if you were to rout out the back of the sign, say to a depth of 3/8", leaving 2-3" full depth at the sides and ends and leaving a full depth across , say every 12" - so it's like a ladder pattern?

Wouldn't that relieve some of the tension in the wood and reduce cupping? In other words rather than fight the pressures causing the cupping by using just a brace, you reduce the cupping pressure by relieving some of the stress. On Andy's drawing the top face (with the arrows) would have to be the front of the sign.

I'm not sure the quarter sawing question answers the splitting question as oak is prone to so much weather related movement.

All the best

Steve

 

[neilr]

Excellent advice folks - many thanks to all who replied. I'm tempted to go with the Osmo finish.
A few replies mention oak being particularly prone to movement when used outside like this. If I didn't use oak for this sign, whet would be the preferred timber (and finish). I think it's quite dificult to get good hard mahogany nowadays, and iroko I hear has nasty dust (more so than other hardwoods?). I'm not wed to oak if there is a more suitable timber for this.

Thanks again.

Neil

 

[Steve Blackdog]

Excellent advice folks - many thanks to all who replied. I'm tempted to go with the Osmo finish.
A few replies mention oak being particularly prone to movement when used outside like this. If I didn't use oak for this sign, whet would be the preferred timber (and finish). I think it's quite dificult to get good hard mahogany nowadays, and iroko I hear has nasty dust (more so than other hardwoods?). I'm not wed to oak if there is a more suitable timber for this.

Thanks again.

Neil

Have you thought of Sapele? - it seems relatively stable in the weather. I've not had much to do with Idigbo, but it is a lighter coloured hardwood - perhaps an acceptable oak substitute.

 

[Phil Pascoe]

a neighbour of mine advised me to use idigbo if i wanted wooden window frames--and he ran his own joinery for years, and now teaches wood machining at a local college.

 

[neilr]

Going with oak in this instance as that's what I have in stock and there's nowhere locally where I can get the alternativee hardwoods. However I will be buying in idigbo and sapele - also iroko to try out some samples and go from there. It's all been very good advice for me though and all assigned to the grey matter for future use.

Many thanks to everybody who replied.
Neil