What is the sticky stuff on the wood?
The sticky residue you see on the surface of wood and wood products is resin. Resin occurs in wood either as a natural substance or as a result of injury to the tree during growth.
When it occurs naturally, it adds to the character of the wood. In these circumstances, it may occur in sufficient quantities. Wood with naturally occurring resin is used where the effect of the resin is to be exploited, for example where increased natural durability is required.
The other form of resin is the result of physical or natural injuries to the tree. These can occur on branches that are not yet fully fused to the main trunk of the tree. Occasionally, physical injury can cause resin pockets to form in the tree. Resin also comes out during an invasion of bark beetles to eliminate the attackers.
The wood we use to make general joinery is usually selected to be free of naturally occurring resin. However, resin created as a result of injury to the growing tree may still be present.
After processing, the wood no longer produces new resin. During the maturing, drying and selection process, wood with obvious resin galls is discarded. The standards for joinery allow the cutting out and repair of resin galls in joinery products where practical.
Appearance of the resin
There are various reasons for the unexpected appearance of resin at a later stage of wood products . Resin contains naturally produced oils. Over time these evaporate and the resin hardens. If the wood is left for a long period of time, all the resin in each piece of wood will harden completely. However, in modern joinery, it is not practical to store wood for this period. Drying in the kiln, which is used to reduce the moisture content of wood, helps to draw out the resin that has come to the surface and remove it at an early stage.
Softwood intended for outdoor use is often treated with preservatives, usually an organic solvent-based preservative. This process is not affected by the presence of resin, nor does it attack the resin.
The problem of resin exudation usually only occurs after the surface coating has been applied to the finished wood product and may take several years to appear.
The machining process can expose subtle pathways to the resin pockets. Specific parts of a joinery product can be heated either by local heat sources or by direct sunlight. Internally, controlled heating systems can raise the surface temperature of wood components just enough to draw the resin to the surface. Externally, a combination of direct sunlight and dark surface treatment systems can also raise the temperature and cause the resin to flow.
The resin leaking out can show up on the surface in two ways. If a microporous paint or stain has been used, the resin may bleed through the surface. If a non-porous paint has been used, the resin will lift the paint off the wood, exposing the surface underneath. In both cases, the wood can be left until the resin flow has stopped, but this is not a very satisfactory solution if the wood is exposed to the elements or the aesthetics of the structure are destroyed.
How to remove sticky residue from wood?
Removing resin from cladding, windows and doors and other exterior wood products .
It may be possible to add more resin to the surface by washing away the existing resin and gently heating the surface of the wood with an electric lamp or heat gun. Gently wiping the surface with a lint-free cloth dampened with methylated spirits will remove the surface resin. If the wood is coated with microporous paints, this procedure, if carried out carefully, can avoid the need for repainting.
If no microporous paints or stains have been used, the best solution is to allow the resin to cure on the surface. This can then be removed by gently scraping. During the time the resin is moving, the finishing system continues to perform its function and does not need to be repainted until the cured resin has been removed.
If the finish system has been lifted, a different solution must be used. It is preferable to leave the resin as it was until it has hardened, then scrape it off and refinish the bare wood surfaces with a full finish system. However, this method can only be used if the wood is not exposed to the elements. Protection of the exposed wood must be carried out as soon as possible to prevent moisture levels from rising and subsequent coats from coming off due to excess moisture.
There is no guarantee that resin will not leak out and damage the decoration of a piece of furniture.
In the past, shellac knot solutions were used, but these are only designed to reduce contamination of the coating system, not to eliminate it. They can never be completely successful because modern coating systems are designed to be flexible. Either they do not adhere to the knot site or the resin forces the decor to detach from the wood.
Once the resin has reached the surface and the above measures have been taken, the problem is unlikely to occur again.