Application processes

For the treatment of building and constructional timbers, there are a range of different processes.

Dipping

The dipping process involves the wood being completely submerged in a tank of preservative solution. Results for preservative retention depend on dipping times and solution concentration, and are also influenced by wood types and moisture levels.

When using water-soluble preservatives, the wood moisture level must be below 30% when treatment begins. For preservatives containing solvents, it must be below 20%.

The mean preservative retention for constructional timber with a moisture level between 20% and 30% is calculated from spontaneous absorption and solution retention over time. There are considerable differences in spontaneous absorption between planed and rough sawn wood:

Wood surface

Spontaneous absorption
[g/m2]

planed

85±15

Rough sawn

200±15


Further detailed information and graphs/diagrams on solution retention depending on wood type, wood moisture and time can be found in the leaflet “Processes for the treatment of wood with wood preservatives, part 2: non-pressure processes” published by the German Society for Wood Research (DGfH) in Munich (Deutsche Gesellschaft für Holzforschung)

Immersion

A special dipping process in which the wood remains immersed in the preservative solution over a period of one or several days.
Due to this long exposure time, immersion also enables the treatment of wood with moisture levels of between 30% and 50%.

Spray treatment

Preventive spray treatment of timbers is carried out in stationary plants. Spray tunnels enable the preservative to be applied evenly and economically with very little wastage. In the main, timbers of use classes 1 and 2 with moisture levels of < 20% are treated in spray tunnels.

Manuel spraying today should only be used as a curative measure applied to wood already in use.

Brushing

This simplest form of protective treatment is the one mainly used in DIY. Commercially, the cost and time involved means this process is only used in special cases, for the treatment of large constructional components (e.g. structural laminated timber) or the remedial treatment of cross cut ends.
To achieve the required retention levels, several coats are usually necessary.

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