Gluing a Merbau frame

[malb]

I have prepared a door frame for a brick letterbox from 70 x 19 (mm) merbau decking. I chose merbau for its good endurance when exposed to weather as I want it to last for some time as it needs to be sealed into the brickwork and would be difficult to replace in the future.

I started gluing the frame together yesterday using polyuerethane glue (small bottle purchased a few weeks ago and used to glue the hardwood filler blocks into a folded SS door skin, so fairly fresh.) My understanding is that poly glue cures by reacting with moisture in the timber.

I anticipated that I would need to keep the joints clamped for a few hours while the glue dried. At four hours the squeezeout had bubbled and frothed but not set hard, so the unit was left clamped overnight. Looked at it again this morning at about 23 hours after glueup and clamping, removed clamps and checked. The joint was capable of supporting its own weight when the peices were held in one hand, but as soon as I held the part with a hand on each joined part the joint failed.

I have searched the forums under ' merbau glue' and come up with one piece of advice from

Make sure the timber is not Merbau. It is an oily timber that is a tad awkward to glue.
If it is, wipe each side with General Purpose thinners first, then glue.
I hadn't come accross that information prior to attempting to glue the joint so hadn't followed the advice. My other concern is that the timber may not have a sufficient moisture content to properly cure the glue. If so, would cleaning the joint with GP thinners and then dampenining with water improve the situation or just produce a weak joint by activating the glue before it has a chance to penetrate into the timber to produce a decent joint.

Any advice based on experience would be appreciated.

Thanks

Mal

[mic-d]

Hi Mal,
What sort of joints are you gluing? Your problem is probably not to do with moisture, if it is foaming that's a good indication that there is enough moisture. But it couldn't hurt to wipe the merbau with a damp (not wet) cloth prior to glue up. If you've selected a good joint and sounds like there is enough moisture the other thing is it may be the glue itself. Do a search for polyurethane glue failure on this site and the web and you'll see there are lots.
If you are happy with the joints you've chosen then I would use titebond3 instead.

Cheers
Michael

[Skew ChiDAMN!!]

Also depends a lot on what type of joint you're using.

eg. A mitred "picture frame" joint look pretty but is inherently weak. Especially as it's end-grain to end-grain. These really need a biscuit or floating tenon to work well. (Better still is to use a mitred tenon joint, but that's getting really fancy.)

I'd also suggest Titebond III instead.

[malb]

Thanks guys, joints are a rebate joint with the rebate half depth to match the rebate that the door fits into. Previously attempted to do the frame with housed joints but ran foul of the rebates for the door recess, will clean up what I have and retry with tb111.

[soundman]

Just a thaught.....ya know this merbau will blead like all get out if you don't seal it properly.

anyway, there have benn heaps of reports of problems with poly u.

My choice would be marine epoxy..... but remember too if you are gluing timber to stanless you will have differing rates of expansion.

So my inclination would be epoxy the frame together and then glue it down to the stanless with sicaflex or similar (flexible).

If ya realy want it to last..I'd do an epoxy impregnation with a UV inhibited varnish over......the varnish will still die a death eventually all clear finished do exposed to UV.... but the epoxy will definitely help durability.

cheers