This is a very common problem that needs to be carefully considered when deciding upon what coating system to apply. Obviously using timber with little or no knots present is a preferred option, but in instances where use of timber without knots is not possible the following coatings solution is recommended.
The use of a two component polyurethane primer such as Induline ZL-550 to minimize and slow down the onset of knot staining is a solution. In extreme circumstances when the coating becomes very warm or there is a lot of resin present in the knot then knot staining will occur even when such a primer is used. While there is no guarantee against knot staining Induline ZL-550 has been proven to be very effective against it in most cases.
This is because many of the common hardwoods used in the UK joinery industry contain water soluble tannins.
These tannins can then migrate from within the timber and into the water based coatings causing the resulting discolouration of the finish paint, which is particularly noticeable if the finish coat is white in colour. In order to prevent this from occurring it is important to isolate any tannins that may be contained within the timber. A specialist coating such as Induline ZW-400 mid coat is able to perform this isolation task.
Coarse grain timbers often have pores in the surface and when the paint does not fill these pores air from them escapes through the drying film causing bubbles to appear. Try using pore filling primers such as Induline GW-208 and mid coats such as Induline ZW-400 or Induline ZW-502. These products flow into the pores replacing the air pockets.
In the first instance, the selection of good quality timber and attention to surface preparation using the correct initial sanding procedure will help to prevent this from occurring.
It is still possible however that a variable degree of surface porosity will cause uneven absorbency of the translucent basecoat stain resulting in a ‘patchy’ appearance.
The use of a clear impregnation such as Induline SW-900 provides a solution by taking up some of this variable absorption.
The ambient conditions in any joinery production facility can have a very significant effect on the final finished quality of the coatings. With the majority of high quality coatings now being water-borne it is important that the water contained within them is able to dissipate into the surrounding atmosphere in a controlled and efficient manner.
After the initial period of drying when the coating should be allow to flow out of humid and still conditions, the perfect drying conditions are good air movement and a degree of warmth. Just like when washing is on the line a warm windy day creates faster drying than a hot humid day.
Cold conditions allow the air to hold high levels of humidity so the use of heaters to elevate the temperatures in the drying areas is a key issue. Avoid heaters that directly burn fuels producing additional moisture. The simplest rotary ceiling mounted type fans are often very effective as they create air movement but also drive warm air (that has a tendency to rise to the ceiling) back down to floor level.
Water-borne top coats often contain a small amount of solvent (called coalescing solvent or co-solvent), to help them form smooth films when drying. If the coating is not sufficiently dried or an appreciable skin forms before the coating underneath has had time to dry the co-solvent remains in the finish. This co-solvent goes milky when water gets into the surface of the film. Try to ensure coatings are dried properly in the factory before putting them out in the rain. In most cases the co-solvent will dry out of the coating film over time and the effect will start to dissipate and eventually not reoccur.
Timber movement is usually caused by a large change in the moisture content of the timber or the use of a timber that is unstable to moisture related movement. Joinery being coated too dry and then being put in UK wet weather is often a cause of movement. Try to coat joinery between 13 and 16% moisture content if possible.
Joinery being placed in a very wet environment with no ventilation is another common reason for excessive movement, try to avoid leaving windows and doors closed on new build sites where there is wet plaster, or concrete as the moisture from the drying material has no place to go apart from into the timber.
Finally ensure the timber coating is properly cured before sending joinery out on site. For Induline DW-601 Aqua Stop, 48-72 hours is enough time to dry the coating sufficiently to protect the timber from excessive water uptake through the non cured coating. Some acrylic primers can take as long as 14 days before they can be placed in wet weather conditions otherwise severe moisture uptake can be the result.
Clear coating systems are available but they only have limited durability.
Under exposure to ultraviolet light from the sun clear coating systems have a tendency to become brittle. Additionally, as a result of the exposure to UV the surface of the timber is also damaged, although this may be difficult to see with the naked eye. This then results in the surface of the timber becoming de-stabilized which in turn may cause the clear coating to lose its adhesion and thus become detached on a localized or widespread scale.
It is possible to prevent this from happening so quickly by introducing clear UV light absorbing additives into the coating so that they can delay the onset of the inevitable break down of the coating, this can extend the life to perhaps 2 or even 3 years but the cost of such additives can make the clear coating system prohibitively expensive.
For further information on the possible clear coating options we recommend you talk to one of the Remmers technical sales consultants.
The pigments used in exterior coatings are able to absorb and reflect UV light and reduce or slow down the damage to the surface of the timber.
This presence of pigments in an exterior coating effectively results in a more durable system with a much longer lifespan of up to 10 years before re-decoration in many cases. As a general rule, the more pigment that is in the coating the longer it will last. The exception to this is very dark coatings which can age prematurely because of solar heat gain. Light coloured opaque coatings do not suffer from excessive solar heat gain and tend to be the most durable of all.
Light translucent coatings are the least durable because they do not have sufficient pigment to absorb high amounts of UV light.
The end grain section is the most vulnerable area of any joinery item. The absorption of moisture is many hundreds time greater through the cut end grain sections than through the face of the timber. When problems are experienced on exterior timber constructions they are frequently observed near to end grain sections due to moisture uptake. Good sealing of these areas is one of the best forms of joinery protection. Exposed end grain that will be present at the end of sashes and door leaves need to be specially protected by using end grain sealers. The spray coating requires this supplemental assistance. Remmers offer a brush applied coating called Induline SW-910 which is a water tight product especially formulated for sealing the exposed end grain. It is applied between the base primer and the mid coat layer by brush and dries quickly to allow for subsequent spraying.
For efficient spraying of joinery products it is necessary to use a pneumatic pump to apply the product. Remmers recommend the use of an air assisted airless spray pump of which there are many makes available. These pumps can deliver sufficient quantities of paint or stain to coat joinery items in a matter of minutes. Some companies try to use conventional systems such as gravity fed guns or pressure pots and while these may be ideal for lower viscosity products used in the furniture industry they do not deliver product fast enough to coat large items quickly and efficiently. They also struggle to atomize some of the higher viscosity paints and stains used for exterior windows and doors.
For further advice on the most suitable spray pumps — contact a Remmers technical sales consultant on 01293 594010 or firstname.lastname@example.org