Adhesive choice for an outdoor bench seat
A landscape contractor orders six spotted gum bench seats for a coastal park. The seats will be fully exposed to sun, salt spray, and driving rain, with no roof or cover. The workshop normally uses a cross-linking PVA for interior work and has a drum of it in stock. The specification calls for an adhesive suitable for exterior exposure, but the supplier's data sheet for the PVA states it is for interior use only and not recommended where water pooling occurs. The seats are laminated from 45 mm sections, and the client wants a 10-year service expectation. The workshop also has a two-part resorcinol adhesive available but it cures dark and requires a minimum workshop temperature of 20°C; the current workshop is 14°C overnight.
1. Which adhesive should be used, and what does the temperature constraint require?
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The cross-linking PVA must not be used. Its data sheet excludes exterior exposure and water pooling, and a fully exposed coastal bench will see both; a 10-year expectation is incompatible with an interior-grade adhesive regardless of how well the joints are fitted. The resorcinol adhesive is the appropriate choice because it is a recognised exterior-grade structural adhesive, but its 20°C minimum cure temperature is not met at 14°C. The workshop must either heat the glue-up area to at least 20°C for the full cure period, or postpone glue-up until conditions allow. Curing at 14°C risks incomplete cure and early joint failure in service - the failure would appear after installation, far from the workshop, which is the worst outcome for both the client and the contractor's reputation.
2. The contractor asks to 'just use the PVA and seal it well'. How should the workshop respond?
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Decline, and explain why sealing does not solve the problem. A surface coating slows moisture ingress but does not prevent it; end grain, joint lines, and any coating breakdown will admit water, and pooled water on a horizontal seat is the exact condition the data sheet excludes. Once water reaches an interior PVA glue line, the bond softens and creep or delamination follows, and the failure is not repairable in a way that restores the original strength. The workshop should offer the resorcinol option with the temperature control cost included, or propose a design change - for example, mechanically fixed slats with drainage gaps and stainless fasteners - that reduces reliance on the adhesive. Either way the decision should be recorded in writing so the contractor cannot later claim the workshop chose the adhesive.