Application and drying conditions
The correct film formation, curing, adhesion, and absorption of timber coatings are all dependent on and heavily related to the conditions under which the coatings were applied and dried. The long-term performance and aesthetic qualities of the coating system are therefore linked to the initial application and drying. Improperly cured coatings can be more susceptible to damage from moisture, which can be particularly crucial on building sites where joinery items become wet as a result of exposure before installation, and certain wet trades that are in progress.
Problems caused by incorrect drying can be as follows:
- Bubbling of the topcoat in wet conditions
- Milky looking areas in translucent and dark opaque finishes
- Excessive water uptake through the coating system in wet conditions
- Poor adhesion and absorption of the coating system
Low temperatures can cause a range of problems such as:
- Improper curing – the polymer used to bind the coating film together and hold the pigments in place coalesces to form a continuous protective layer. This process is assisted by the coating film being at a sufficiently high temperature. Also, some constituents in the film cross-link with each other. This cross-linking process requires a certain temperature to take place. If the cross-linking does not take place, the coating does not build up the necessary protection against moisture
- Cold joinery items can result in surface dew formation which will hinder absorption of the coating
- Cold joinery items will lower the temperature of the applied coatings, making them more viscous and less likely to adhere to and absorb into the substrate
- Cold air temperature and storage conditions will make the coating more viscous, resulting in reduced flow and lower absorption
High temperatures can also result in problems:
- Orange peel effect – before the coating film has had sufficient time to flow out to a smooth film, the skin dries when the coating is still showing the undulations left by the spraying process. This leaves an orange peel effect in the surface
- Mud cracking – this occurs when there is rapid skin formation over a liquid layer of paint, which can happen in very warm drying areas. The net result can be that the liquid or semi-liquid layer is still moving and drying under the skin, causing cracks to appear in the film
- Dry spray effect – if the drying conditions are very warm and the humidity is too low, one side of the joinery item has started to dry while the other side is still being sprayed. The wet spray layer lands on the dry layer opposite and forms a rough surface that does not flow out
High humidity during the drying process can result in:
- Preventing moisture from escaping the coating. This slows down the drying process as the moisture has to escape from the wet film. High humidity can be a result of a low temperature. The two are linked as warm air usually holds more water vapour, resulting in a low humidity.
Low humidity during the application process can also result in:
- Spray finishes suffering from orange peel and potential mud cracking as described above, due to rapid skin formation. A humidity of above 60% at the time of application allows the coatings to flow out better, giving it a much more attractive and smooth finish
- In the case of flow coating or dipping products, a lower humidity causes spray finishes to run off very inefficiently, giving it a worse appearance
Ideal drying conditions are as follows as shown in this overview:
Once the drying process is taking place, lowering humidity and increasing temperature will reduce drying times. Forced drying of temperatures between 25°C and 35°C and a relative humidity of around 30% will promote rapid drying. Air movement to remove local humidity is very important. An industrial fan will help with this and is highly recommended.
Assisted force-drying can make very rapid drying and curing a possibility. An example of this is catalytic IR (infra-red) drying equipment. When using this equipment, our Hydrolux exterior coating system can be dried at a rate of 20 minutes per coat. We have tested this with one of the leading manufacturers of this equipment.
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On windows and doors, where the wood suffers from lower movement, thick-layered stains or varnishes can be used as they can cope with the smaller movement. Anker Stuy woodstains do not form a thick film layer, as this will peel and flake on items where there is appreciable movement. Our woodstain products have a lower film thickness, so they will not peel and flake as the wood moves and flexes.
- Obtain a suitable temperature and humidity gauge to measure conditions in the factory. Data logging equipment should also be available to monitor temperature and humidity fluctuations in the factory, overnight and during the weekend to give a better idea of what the conditions are
- Coatings can take 2-3 days to thoroughly dry or cure. We recommend you keep items above 15°C inside for a period of 72 hours prior to exterior exposure
- Store coatings at a suitable temperature. Never allow coatings to be stored under 5°C. Try to ensure they are at 15°- 20°C for application. Keep product on shelves or pallets and off concrete floors
- Try to ensure the timber that is being coated is at a temperature of 15°- 20°C
- Try to ensure coating equipment such as spray pumps are at a temperature of 15°- 20°C
Curious to see how the cold and heat affect our Hydrolux filling primer? Watch the video below to find out!