Best Glue for Australian Hardwood

What is the best adhesive for gluing Australian hardwoods such as Merbau, Jarrah and Blue Gum?

For most hardwood joinery, Aquadhere Exterior Wood Glue is the right answer. It is a cross-linking PVA and it forms a stronger bond than standard interior wood glue — which settles the yellow-versus-crosslinked question you have been reading about online. Aquadhere Exterior is the cross-linked product in the Selleys range.

But for tannin-rich or oily timbers — and Merbau and Jarrah are exactly that — Selleys recommends a polyurethane adhesive instead: Aquadhere Durabond. The tannins and oils in these timbers can cause a discolouring of PVAs. That reaction does not occur with Durabond. Durabond also has the benefit of being even stronger than the cross-linked PVA, and in most cases you will find the wood failing before the adhesive does.

The two products side by side

Property	Aquadhere Exterior	Aquadhere Durabond
Technology	Polyaliphatic cross-linking PVA	Solvent-free, moisture-curing polyurethane
Colour	Off-white, dries clear with a yellow tinge	Amber, dries yellow opaque
Coverage	Approximately 5 m²/L	—
Working time	—	About 10 minutes, depending on temperature and humidity
Clamp time	30 minutes softwoods; 1 hour hardwoods	Clamp firmly until cured, approximately 4 hours
Maximum strength	24 hours	Up to 24 hours, depending on humidity and material porosity
Heat resistance	—	Maximum 140°C
Application temperature	—	Above 5°C
Packaging	250 mL, 500 mL, 1 L, 4 L bottles	460 mL bottle

Indicative data only, not intended for the preparation of specifications.

What the tannin problem actually is

This is the reason the recommendation splits by species rather than by application, and it is worth understanding.

The Aquadhere Exterior data sheet states plainly that Aquadhere will stain with a brown or black colour if contaminated with rust (iron oxide) or in the presence of tannins in many woods. Merbau in particular is notorious for tannin bleed — it is the same chemistry that leaves brown runs down a wall below a new Merbau deck after the first rain. In a glue line, that reaction shows up as a dark stripe along every joint, which on a finished piece of furniture is not something you can sand out without cutting the joint open.

Durabond is a polyurethane and does not react this way. That is the whole basis of the recommendation — it is a staining question first and a strength question second.

Working with Durabond on dense hardwood

Durabond behaves differently from a PVA in three ways that catch people out.

It expands and foams as it cures. The data sheet says to apply a thin layer to one surface only. Applying it liberally the way you would a PVA produces foam squeeze-out along the joint, which is messy and weak. A better bond comes from tighter clamping, which controls the foaming.

It needs moisture to cure — and dense hardwood does not have much. The data sheet's instruction is to wipe very dry porous surfaces with a damp cloth for 1 minute before bonding. Kiln-dried Jarrah in a dry shed is exactly that case. Conversely, high humidity and damp surfaces cause increased foaming and a weakened joint, so this is a middle ground you are aiming for rather than "wetter is better".

It bonds to skin. Wear gloves. This is the first line of the directions, not a footnote.

How to glue a hardwood joint with Durabond

1. Wear gloves — the adhesive bonds to skin.
2. Make sure the surfaces are close fitting, clean, dust free and grease free. One surface should be porous.
3. If the timber is very dry, wipe it with a damp cloth for 1 minute before bonding. For smooth non-timber surfaces such as glass, plastics or metals, sand or roughen them first.
4. Apply a thin layer to one surface only. The glue expands and foams as it cures, so thin is correct.
5. Assemble within 10 minutes.
6. Clamp firmly until cured, approximately 4 hours. Tighter clamping gives a better bond and less foaming.
7. Leave it to reach maximum strength, attained in 24 hours depending on humidity and material porosity.
8. Wipe the nozzle and seal the bottle tightly. Store in a dry place — the product is humidity sensitive.

Note on clamping: this page has previously said the bonds must be tightly clamped for up to 12 hours. The current Aquadhere Durabond technical data sheet gives approximately 4 hours clamping, with maximum strength at 24 hours. Longer clamping does no harm and is sensible on a highly stressed joint, but 4 hours is the published figure.

Limitations

• Aquadhere Exterior: not recommended for joints subjected to constant high stress; not suitable for below waterline applications or continuous submersion; avoid contaminating it with iron and steel (nails and similar), which will cause brown or black staining.
• Aquadhere Durabond: waterproof but not for permanent water immersion or below waterline use; not for polyethylene or polypropylene plastics (test other plastics first); not recommended for structural applications; requires atmospheric moisture to cure, so use may be restricted in dry conditions; cannot be used below 5°C.
• Neither is a substitute for mechanical fixings on a structural joint.

Clean up

Aquadhere Exterior: uncured product removes with water; cured product can be sanded or scraped. Durabond: clean tools and uncured product with acetone or turps immediately after use; cured Durabond must be scraped, cut, chiselled or sanded off. Durabond is sandable without clogging sandpaper or the timber the way other PVA glues do, and it is paintable and stainable after sanding.

Common questions about gluing Australian hardwoods

Is Aquadhere Exterior the crosslinked PVA?

Yes. Aquadhere Exterior is a polyaliphatic cross-linking PVA, and it forms a stronger bond than standard interior wood glue.

Why not just use the PVA on Merbau?

Because tannins and oils in these timbers can cause discolouring of PVAs, showing as a dark line along the joint. Durabond does not react that way.

Which is stronger?

Durabond. Selleys notes it is even stronger than the cross-linked PVA, and in most instances the wood will fail before the adhesive does.

How long do I clamp a hardwood joint?

Aquadhere Exterior: 1 hour for hardwoods, and do not stress the joint for 24 hours. Durabond: approximately 4 hours, with maximum strength at 24 hours.

Can I use Durabond outdoors?

Yes — it is suitable for both interior and exterior applications and is waterproof, though not for permanent water immersion.

Will Durabond fill a gap?

It expands as it dries and can be used for gap filling. Aquadhere can be mixed with sawdust to fill small gaps.

Can I stain or paint over the glue line?

Durabond is paintable and stainable after sanding. Aquadhere Exterior dries clear with a yellow tinge.

Does Durabond work on wood to metal?

Yes. It bonds wood to almost anything, including MDF, metal, ceramics, glass, masonry and most plastics. Sand or roughen smooth surfaces first.

What about screws and nails in the joint?

Keep iron and steel away from Aquadhere — contamination with rust causes brown or black staining. Durabond is not affected this way but is not recommended for structural applications either.

How do I get dried glue off?

Water for uncured Aquadhere; acetone or turps for uncured Durabond. See also removing dried Aquadhere from clothing.