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PVAc, PU, and traditional mechanical joints — how to choose correctly
A joint is only as strong as its weakest link and in the carpenter's trade the choice of glue and joint type is rarely accidental. The right choice depends on where the piece will be used what moisture it is exposed to and what load the joint must withstand over the years.
| Glue type | Properties | Typical use |
|---|---|---|
| PVAc (white wood glue/carpenter glue) | Standard glue type, good strength in dry or moisture-resistant classes | Furniture, indoor carpentry |
| PU (polyurethane adhesive) | High strength and good moisture resistance can glue different materials together | Windows outdoor structures mixed materials |
| Epoxy | Very high strength, can combine e.g. wood with metal or plastic | Repairs, special tasks with combined materials |
PVAc glue is divided into moisture classes. D2 glue is intended for indoor use in dry environments, while D3 glue is more moisture-resistant and is chosen for moisture-exposed environments such as bathrooms and kitchens. Choosing the wrong moisture class can result in a joint that fails long before the rest of the workpiece is worn out.
A wood moisture content of approximately 7–15% is best suited for bonding with PVAc glue while PU glue typically works well at 12–20% wood moisture. A PVAc bond starts to cure within 30–60 minutes but only achieves its full strength after approximately 24 hours — it is therefore important that the joint is not fully loaded before the glue has had time to cure completely.
Long before modern adhesives existed, the carpenter held pieces together with purely mechanical joints. Mortise and tenon joints provide a large glue surface and mechanical interlock, while dovetail joints — the tooth-shaped joints where two boards interlock — are known for their great strength in drawers and boxes.
For an outdoor window corner, a joiner would typically choose a mortise-and-tenon joint glued with PU glue or similar moisture-resistant glue. For an indoor drawer in dry surroundings, a dovetail joint glued with ordinary PVAc glue is often sufficient. The right combination is about understanding what the item will really be exposed to in its life after the workshop.