David WE Roberts wrote:
On Sat, 16 Dec 2006 15:29:09 +0000, The Natural Philosopher wrote:

Richard Downing wrote:
The Natural Philosopher wrote:
wrote:
Richard Downing wrote:
stevesmith wrote:
I need to join some tanalised planks of wood edeways. The wood is 45mm
wide.

Do I have to plane the edges down to untreated wood and then water
proof glue them together, or can I glue tanalised without planing ???
snip
My view is that in exterior situations, glue in any shape or form is
pretty inappropriate..it hasn't got the tensile strength, and wood will
split itself, let alone glue, when you get wide humidity variations.

Strange, then, that a lot of wooden boats are built using glue.

Perhaps the difference is that boats tend to have other protection such
as paint or varnish, but I suspect a lot of outdoor woodwork such as
hardwood patio furniture is also glued.

Indeed. And they use very stable wood..usually plywood..which has its
own very carefully formulated glue in it..never tanalised building lumber!

AND they are definitely given many many coats of very tough paints and
varnishes, which need regular maintenance.




My old clinker built dinghy had various glued bits, and had to be soaked
at the beginning of the season to become watertight, so I don't think that
having the wood constantly damp rules out gluing either.

If it needed ti be soaked to make it watertight, then the planks weren't
glued together were they? and you have proved my point that planks move
hugely under humidity changes opening up gaps..no glue would seal those
clinker planks together..you caulk a and wet and hope..

The frame? well a GODD piece of planed wood with proper joints and a
synthetic resin glue will take immersion for a while.. but traditionally
all boat frames were 'one piece' wherever possible..even down to
selecting curved branches for the parts.and tenoned and pegged..

I think the first glues that really worked for boats were cascamite and
aerolite 306. Glues that set and were of a plastic variety..but t even
they won;t hold up it the wood becomes sodden.




Perhaps gluing softwood with no protection against becoming waterlogged
then bone dry on a regular basis might destroy a glue bond because the
glue will not expand and contract with the changes in moisture.

On the other hand, marine ply is glued together, is it not?
Also exterior grade ply?

They hardly expand at all. The issue there is the cross graining..you
get about 3-5% expansion across the grain, and almost none along ot.
Cross bonding locks the cross grain stiff in place hard..those plies are
made under pressure and heat on presses using very tightly controlled
gluing. They are very stable. Probably less than 0.1% movement in any
direction once made.


Perhaps the wrong wood and/or the wrong glue are being used in this
situation?


Yup. Tanalised lumber is simply crap wood given an anti-fungal treatment
on the surface layer. Its got no water rejection properties at all and
it moves like a bitch under humidity cycling. Depending on how its fit
it can warp,cup and shrink all over the place. Tying it down will like
as not result in in splitting if the section is small enough..or even if
its not.


Cheers

Dave R