The log cabin has blackened wood on the outside. How can it be cleaned?

"The round log cabin has blackened wood on the outside in places. Can it be cleaned?" This is the question asked by someone interested in buying the log cabin. The message mentioned that it had never been lived in and I assumed that the wood had not been protected at all on the outside. As a result, the blackness was from damp, mould and the natural change in colour of the wood under UV rays. The best way to achieve clean wood in this case is sandblasting, with the recommendation to call a company specialising in such services. There are blasting guns and machines on the market, some at very affordable prices, leading many to believe that they can easily do it themselves. Sandblasting, however, is a complex process, and if it's the wood that needs to be sandblasted, things get even more complicated.

Wood sanding

Why is sandblasting wood more complicated? Because unlike other materials that are homogeneous, wood is a non-homogeneous material, and this makes it very vulnerable. The sanding of wood must be done with great care, taking into account the species, structure, age of the object or the conditions in which it has stood. The risk is that during the process the wood will be irreversibly damaged and thus the object compromised. We have seen this here, when old doors were destroyed by being sandblasted by someone who had never worked with wood before. That's whySandblasting is not used to restore wooden heritage objects and is not recommended when the object is weakened by the weather.

Sandblasting is used in the cleaning, polishing and structuring of various materials, from those used in dentistry to concrete, stainless steel or plastic. It is also used for wood structuring and even to create art objects. The process involves the abrasive material, the air or water jet that drives the material and the machine that makes it all possible. In order for the wood not to suffer, the quality and aggressiveness of the abrasive material, the pressure used, the distance from the gun to the object and the interaction between all these must be taken into account. Also, as a process that can generate waste and affect air quality by loading it with very fine particles, choosing natural materials and methods that are as environmentally friendly as possible is also important.

Abrasives and blasting machines

The first material used in the blasting process was sand. In the meantime, things have evolved, the materials used have diversified and are used more and more in a targeted way. We now find as abrasive material glass, ceramic or plastic beads, normal or stainless corundum, grit from metallurgical or glass slag, silicon carbide, sodium bicarbonate, salt, vegetable granules. The main difference between them is the hardness set according to the Mohs scale which measures the ability of materials to scratch and be scratched.

For wood, materials that are higher than 10 on the Mohs scale should not be used, and this includes baking soda, glass, ceramic or plastic beads, granulated pumice stone, vegetable granules (walnut, corn). Bicarbonate is preferred because it does not destroy wood, is non-toxic, 100% environmentally friendly, water soluble (the resulting solution can be discharged into the sewer) and can be used with an air blaster.

An increasingly used material lately is dry ice or dry ice. Use alone or in combination with other abrasives. Special machines are needed for such blasting, machines that generate ice particles. If less aggressive blasting is needed, machines can be equipped with systems that break up the ice particles, making them smaller and less aggressive.

The most commonly used machines for sandblasting are those with compressed air. The pressure required is between 2 and 7 bars. There are also blasting machines that use dry steam or water. They have the advantage of greatly reducing the concentration of dust in the air, and the resulting waste is also reduced. However, care must be taken when using them on wood as water can cause problems for the wood. The distance from the gun to the object also influences the quality of the blasting. By varying the distance and pressure, the best combination can be found to achieve the desired blasting result.

Cleaning weathered wood

If the wood is not protected with materials specially made to withstand the outdoorsr, will be affected by humidity and UV radiation. Moisture creates the conditions for fungi and mould to grow and hence the black colour on the wood. It is the UV radiation that attacks the dyes in the wood and breaks them down, giving the wood its characteristic bluish-grey colour. While the change in grey colour is only superficial and the wood comes back clean when it is removed, with mould things are not so simple. If the water doesn't drain away, fungi, mould and insects grow and start to enter the wood. It's the cause of deep damage and rotting. Some species are more resistant to this process, others less so. Unless it is a particularly exotic species, it is good to protect the wood outside, make sure that water drains off and does not stagnate on it and air circulates so that the wet wood can dry.

Black mould can also be removed with special solutions that can be applied with a brush or spray. The part affected by UV rays cannot be treated and must be removed. The wood is only superficially affected, and immediately below this the wood is healthy and clean. Removal can be done by manual or mechanical sanding or blasting. When surfaces are large and difficult, such as log houses, other types of wooden houses or clad areas, sandblasting is the best solution.

Removing old paint layers

If you are sandblasting to remove old layers of paint from doors, windows, frames or furniture, it is very important to know what wood is under the paint. Hardwoods such as oak, walnut or ash are much easier to clean than softwood or lime, which can be easily attacked and structured. A well-done sanding leaves the wood clean and smooth, with no raised grain to feel when you run your hand over it. Before starting the process the objects are checked for weather damage (they are light and break easily). In this case sandblasting is not a solution.

Sandblasting is very effective for carved wood, ornaments, where there are hard-to-reach places. On such objects the usual methods such as sanding, chemical pickling, hot air followed by scraping (methods about which you can find more information here), do not give satisfactory results. In this case too, the fragility of the wood must be taken into account. The best strategy is to remove the paint gradually, by several passes with a sandblasting gun. Varnish, paint or resin layers can be removed in this way. Paint is harder to remove than varnish because of the resistance deposited by the pigments.

I hope you find the information useful. If you have any additions, queries or questions, please leave them in the space below. I will certainly reply.