Following application, waterborne coatings are touch dry very quickly as the carrier (water) evaporates from the surface of the applied coating. However, it takes significantly longer for the coating to thoroughly dry and even longer to fully cure, full curing and cross linking will take place over several days.
The drying of the applied coating system is dependent on many factors including temperature, the moisture content of the timber, applied coating thickness, drying times, elapsed time between applied coats, storage conditions and relative humidity. This means that in times of high humidity or cold damp weather, the joinery may be exposed to weather or excessive moisture from the building process before the drying / curing process is completed. Under these circumstances it is possible that translucent finishes will become milky in appearance and the surface of opaque finishes will blister.
As the coating cures these effects will disappear and is no way detrimental to the long term performance of the coating system, this process can be referred to as coalescing.
The process of drying described enables the coating to maintain its elasticity and therefore its long term durability.
Soluble extractive's and tannin's
Many tropical and naturally durable timbers contain soluble extractives that are released when the timber is wetted by a coating. Such extractives can discolour the coating film, and the effect is most pronounced with traditional water borne coatings. Western Red Cedar is perhaps the most extreme example of a species prone to this type of staining, but grades of Iroko, Idigbo, Sapele, Brazilian Mahogany and Meranti can also show evidence of tannin staining as can individual batches of other timbers and modified timber products. Variation in growing region can produce significant changes in the mobility of extractives found in a specific timber species.
Milky discoloration in translucent systems can happen in cold conditions, when the joinery has been wrapped or is stored in damp conditions. This discoloration will disappear within a few days of the joinery being installed as the residual moisture dissipates to atmosphere and the coating achieves full cure. For opaque systems the issue will show itself as micro blistering, but in severe cases the blisters can be much larger. Again, these will disappear as the moisture dissipates to atmosphere.
Staining of opaque coated joinery is a more complex phenomenon, and generally results from two main sources: soluble extractives or tannins; and resins, both of which are naturally present in timber. Tannins are the main cause of staining in hardwoods, whilst resin exudation tends to be more prevalent in softwood varieties, particularly around knots.
As well as timber species, the severity of staining is also influenced by the following: - growing region; knots; preservation treatment; processing and exposure conditions. Moisture plays a major part in tannin migration through the coating system and site problems occur most often when dry joinery from the factory is exposed in warm moist conditions: the rapid uptake of moisture as the joinery conditions tends to mobilise the resin and tannins whilst heat and sunlight draw them to the coating surface.
With good timber selection, care in the preservation process and appropriate coating selection, the problems can be significantly reduced and in some cases wholly eliminated.