What you need to know if you want to sandblast old paint off wooden doors or furniture

Some time ago I was asked to help repair some old doors that were partially destroyed in the cleaning process. They came from an old apartment that the owner wanted to renovate and save the tall wooden doors typical of buildings built at the beginning of the last century and in the interwar period. He clearly loved the place, and the fact that the doors looked compromised visibly affected him. What had happened? To remove the old layers of paint from the doors, he called in a firm to sandblast them clean. Unfortunately, the sandblasters didn't know much about sandblasting wood and as a result, the doors ended up with very structured wood, punctured in very thin areas and partially destroyed on edges and corners. Despite the result, the owner still hoped to save them, this time proposing to do it all himself.

Sandblasting wood is different from sandblasting metal

Although about wood sandblasting I have spoken of on other occasions, this event gives me the opportunity to highlight the differences between sandblasting metal and wood. This does not mean that it is a simple operation that you can do yourself. You still need to call in the specialists because it involves special equipment and risks during the process. But that way you know what to ask for so you are told up front whether or not they can do the job.

Originally, sandblasting was used to clean metal with sand. So is the definition of the term, if you have the curiosity to look it up in the DEX. Gradually, the process was extended to other materials, and in addition to sand, other materials with different grain sizes and hardnesses began to be used. Wood is a different material from metal, both in hardness and in the fact that it is amorphous, not uniform in composition. If in metal, no matter where on the surface, the strength of the material will be the same, in wood it will be different. On a door, for example, there will be late wood and early wood or knot area, each having a different strength. When sanding wood you may have to deal with hardwood and softwood - that classic division into hardwood (hardwood, as it is called in English) and softwood (softwood) - but also differences in hardness between species of the same class.

Under these conditions, the material used to remove the paint layer and the pressure applied are very important. Depending on them you will either just remove the paint and get a clean and smooth surface of the wood, or remove the paint and structure the wood so that it looks like weathered wood. The choice has to be made very carefully especially when it comes to old wood or objects. For example, sandblasting is not used in the restoration of old wooden objects of historical value. The many variables make the process difficult to control, so it's better not to risk it. It can only be used if refurbishing old doors or furniture.

Abrasive materials used in blasting

Initially used sand for sandblasting, hence the name (sable = sand in French). But sand also comes in many varieties and because of this there are different classifications according to hardness or grain size. In terms of grain size, there is sand with grain sizes ranging from 30 to 220. For wood only 30 to 80 grit is used. In terms of hardness there is another classification. Very often it is used Mohs scale used mainly for minerals, which is based on their ability to scratch or be scratched. Talc, a material that can be scratched with a fingernail, has a hardness of 1.3 Mohs, while diamond has 120.6. Quartz is somewhere in the middle at 72.2. Materials used for wood should not exceed 10 on the Mohs scale.

However, other abrasive materials are also used. For wood, the most recommended is sodium bicarbonate. It's softer, but its crystals are abrasive enough to clean the wood and leave it smooth. There are also specialists who say it's too soft and recommend it more for removing mould or cleaning plastic, but the vast majority agree that it's the best material for cleaning wood. Plastic, ceramic or glass beads, crushed pumice, crushed walnut shells or crushed corn kernels can also be used for wood. All these materials are low in hardness so they do not damage the wood. For example, walnut shells and corn have a hardness of 4.5 to 5 Mohs, plastic beads used to chip wood have a hardness of 3 to 4 Mohs and pumice stone has a hardness of 6 to 7 Mohs.

Regardless of the material used, it is important that it does not deform or dislocate the wood grain. Sandblasting should not be used for cleaning where there is a risk of damage, as in the case of old weathered wood or old and very valuable objects. If you do not want to structure the wood, it is better to clean it gradually, in several passes, so that the wood remains intact at the end.

Why, by sandblasting, were the doors partially damaged? What else can be done?

In the situation presented, the negative result was amplified by the fact that the doors were made of resinous wood. Much softer than oak or even northern pine, spruce (probably) yielded much more quickly, its structure much more evident. Unfortunately, the force of the sandblasting knocked out knots in some cases, ground the edges of the doors at the corners, sometimes even loosening large splinters of wood. Fortunately, not all doors were equally affected.

The doors will now undergo a laborious process of restoration. Holes and gaps will first be filled in and the wood filled in where large pieces have disappeared. Small defects will be covered with putty made from resinous wood dust and aracite. The loose parts will be glued with aracet. Then, theIn order to reduce the difference between late and early wood after sanding, the doors will be sanded all over the surface, along the grain, first coarsely then with finer paper. Only then can they be stained and varnished so that the wood grain is visible, as the owner wanted from the beginning.

I'm sure the doors will look good in the end. Their owner is far too determined to save them.