Spray vs Immersion Timber Treatment: A Process Comparison

Bundled timber is fully submerged in a tank holding the AST solution for a defined contact time, typically 30-90 seconds. The bundle is then lifted, allowed to drip back into the tank, and stacked for surface drying.

Advantages: maximum coverage including end grain, knot fissures and bundle interiors; consistent retention across every face. Disadvantages: slower throughput (limited by tank size and crane cycles), higher chemical inventory in the tank, requires biocide top-up monitoring against dilution from wet timber. Standard at Canadian, German, Austrian and most Brazilian mills supplying premium grades.

Loose or bundled timber moves on a conveyor through an enclosed cabinet fitted with multiple high-pressure spray nozzles. Application takes 5-15 seconds; excess solution drains and is recirculated. The system is automated and integrated with the sawmill green-chain.

Advantages: 3-5x higher throughput than dip; precise chemical dosing; minimal handling. Disadvantages: end grain and bundle interiors receive less solution than the outer surfaces; requires regular nozzle maintenance to maintain uniformity. Standard at modern Scandinavian, New Zealand Radiata and Chilean Pine mills.

Independent retention testing (NZ Forest Research, BRE UK) shows: well-maintained spray systems achieve 80-95% of immersion retention on outer faces, but only 50-70% on internal bundle surfaces. For unbundled, freshly sawn timber moving through a spray tunnel before stacking, performance is comparable to dip.

In practice, this means: spray-treated timber that is stickered (separated by spacers) before treatment performs as well as dip. Spray-treated timber sprayed after bundling performs less well than dip.

For 6-week direct-shipment cargo: either method is acceptable provided mill is reputable and AST certificate is issued. For 8-12 week consolidated cargo (Brazil, Chile, NZ to UAE/KSA): specify immersion or pre-stacked spray. For winter season shipments: either method, with reduced concern. For summer arrivals June-September into UAE, KSA and Qatar: prefer immersion.

TIC Dubai works with sawmills that document treatment method, retention level and bundle-stacking sequence; the certificate is available with each shipment.