Thread: PVA or Epoxy

Chris,

I use epoxy in these situations:

* when I need a gap filling glue
* when the glue is to be tinted
* when the piece is constantly exposed to water, and
* when I want a rigid bond, e.g. preventing springback when laminating curves.

Like the other answers above I’d be interested in the context, e.g. type of timber (if it is timber), whether it requires high shear strength, exposure to water, high/low temperatures etc.

Best regards,

Brian

Originally Posted by Chris Parks

It is in a bent lamination made in Sydney blue gum that was has become unglued by my poor methods or inexperience. The laminations are about 5mm thick and the gaps are on the outside of the lamination. I can easily clamp them closed but it is a very strong timber and the unglued parts are small so not a lot of glue surface for adhesion. It was originally glued with epoxy but I haven't got any and I am tempted to re-glue the unstuck bits with PVA or even super glue if it is suitable.

Now that is a different kettle of fish all together - you are reglueing, not glueing. Changing glue formulation / type now is not advised nor real practical. Use the same adhesive - and preferably the same brand of epoxy.

PVA will have to be applied to clean timber, any epoxy on the surface will have to be removed. Epoxy is one of the few glues that will bond to itself so for this application it is probably the only choice.

Originally Posted by Chris Parks

It is in a bent lamination made in Sydney blue gum that was has become unglued by my poor methods or inexperience. The laminations are about 5mm thick and the gaps are on the outside of the lamination. I can easily clamp them closed but it is a very strong timber and the unglued parts are small so not a lot of glue surface for adhesion. It was originally glued with epoxy but I haven't got any and I am tempted to re-glue the unstuck bits with PVA or even super glue if it is suitable.

I think that will be a tough ask for any glue to hold long term; 5mm is very thick for any bent lamination, but especially in a timber that strong.

Click Bond CB200 is one of the strongest glues I know of; it's used as a substitute for welding in the aerospace industry, but it's mainly for metals and plastics.
CB359 is their product that lists wood as a recommended material and has similar strength according to their documentation (4500psi shear, 10.5kN/m peel strength) with a one week cure time.

I don't think superglue will hold for long and PVA is unlikely to stick to the epoxy already in the join.

Let's not be too pessimistic Actually this was done on the advise of someone else and I have never been happy with the result at all. I would rather have cut it out of a single piece and if I can find some suitable material it would become firewood very quickly.

Not pessimistic, just realistic; there's a lot of tension to hold back there and a good epoxy is probably the only thing that will stay stuck.

While it is not epoxy but Gorilla glue. I can give some observations. Not scientific in any way but just observations.
Two joints made several years apart. One with Gorilla and one with TiteBond III. Both were glued and clamped. Both remained in clamps over night and guessing the clamps were applied for at least 18 hours. It is just my shop style.

Both joints had to be taken apart at the same time and probably 3-6 years after the glue cured.

In both cases the failure during dismantling was wood fiber. In the Gorilla glue joint, about 60% of the joint showed wood fiber failure with the remaining clearly separation of the glue on the two surfaces. In the TiteBond III joint about 95% of the joint showed wood fiber failure. With the wood fiber failure reasonably equally distributed across both pieces in the joint for both glue types.

In all the scientific analysis? There ain't none. Both joints were lap joints using KD 2 x 4s in a right angle shape. IIRC the TiteBond III joint was more difficult to tear apart after the screws were removed.

The method to tear the joints apart was to clamp one part in a vise and use the leverage of the other piece against the joint. Scientific? Nope. Just an observation. Did the wood fiber fail differently? Not that I could ascertain.

As a result of my unscientific observations, I use TiteBond III almost exclusively in my shop.

I have done a lot of epoxying, Chris, and agree it is the best choice for your reglue job. If you can clamp the piece "almost invisible joint tight" then you could use the epoxy straight, otherwise thicken it a little with colloidal silica to enhance its gap filling properties.

In my view any of the recognised marine brands of epoxies are excellent - marine is a very harsh environment, and boat owners are extremely fastideous clients. I use WEST or a local fibreglass shops generic copy. It must be measure accurately - within 3% for optimal strength.

I would suggest the following methodology:

• Clean all surfaces with acetone or MEK removing any loose epoxy,
• Mix up a little more epoxy than you need,
• Take a small portion of that mix and thin with equal volume of acetone or MEK and apply absorption coat to both glue surfaces,
• Wait ten minutes or so, then
• Thicken remaining epoxy (if necessary) and apply to both surfaces and clamp carefully and snuggly, not tightly,
• Wait an hour and tighten clamps,
• Wait another hour and clean off squeeze out. Leave overnight.

I would absolutely avoid "five minute" epoxies such as araldite and anything from a discount store. Pay for a brand name!

In my view slow hardening epoxies are almost always better than standard ones.


Heed the safety instructions; epoxy is a truly nasty substance.

Originally Posted by GraemeCook

Heed the safety instructions; epoxy is a truly nasty substance.

My favourite warning is on Techniglue: Suspected of causing genetic defects

Originally Posted by elanjacobs

My favourite warning is on Techniglue: Suspected of causing genetic defects

I can safely ignore that one.

When I worked in a timber mill years back we used to laminate F17 structural beams out of Vic Ash using a 2 part epoxy mix that was rated for it.

I found it a pain to use and clean up, but it was supposed to be appropriate for the purpose.

Having said that, I'd generally just use glue for most things as others have said!

Epoxy. Etch prime all surfaces to be joined with acetone first.

Originally Posted by riverbuilder

Epoxy. Etch prime all surfaces to be joined with acetone first.

Cleaning with acetone seems to be particularly necessary with Australian hardwoods, especially eucalypts. Spotted gum is notorious for needing it.

Spent a lot of time fitting out yachts. Laminating timber was a big part of the project due to the inumerable curves one encounters. Epoxy is the glue of choice for bent work. As Riverbuilder said, etch prime the faces to be bonded before applying glue. Keep laminations as thin as possible for best stability when removed from the form. 12mm is not impossible but you're pushing it.