Removing Old Coatings with Lye

Removing paint or varnish from various surfaces is often a difficult and complex task. One solution may be the increasingly popular laser removal of coatings (laser ablation) or sandblasting. You can also use wire brushes or sandpaper for mechanical removal.

However, the above methods can be expensive and risky, especially due to the possibility of damaging the surface being cleaned (wood, plastic, plaster). In addition, poorly chosen methods of removing coatings may expose surfaces to unfavourable weather conditions. An example of this is the damage to wall and facade plasters, which can cause the building structure to become wet, thus reducing its mechanical strength. However, an effective and inexpensive alternative to mechanical removal of surface paints and coatings has proven to be the popular caustic soda. The use of lye (caustic soda solution) reduces the risk of surface damage that can occur using other methods.

Properties of caustic soda and rules of behaviour during its use

Caustic soda is a white substance with a crystalline structure. It dissolves well in water, producing a highly caustic solution (caustic soda solution) while releasing large amounts of heat. Sodium hydroxide dissolved in water is an odourless, non-flammable liquid that is corrosive to metals. In addition, the lye reacts readily with metals that are amphoteric, thereby releasing hydrogen gas. Particular care should be taken when in contact with this substance as it appears as dust or vapour and can cause pain, tearing, a burning sensation in the nose and throat, and coughing. In contact with the skin, sodium hydroxide causes chemical burns with possible blistering and necrosis. The substance is particularly dangerous when it enters the digestive tract. It causes burns to the mucous membranes and, in extreme cases, perforation of the oesophageal wall, which may lead to internal bleeding and thus death. Therefore, when handling sodium hydroxide, take great care and use appropriate personal protective equipment such as goggles and protective masks, latex gloves, protective clothing and footwear. It is also important to ensure that the room is adequately ventilated.

What kind of paints and varnishes can be removed with sodium hydroxide?

The most commonly used preparation for removing paints is a 10% aqueous solution of sodium hydroxide. To prepare, measure an appropriate amount of caustic soda and add it gradually and carefully to the previously prepared water, stirring the contents of the container until dissolved. For storing this solution, containers made of plastic or glass are most suitable. The 10% NaOH solution thus prepared is applied to the surface to remove the paint. It breaks down the chemical bonds of the coating, usually due to a hydrolysis reaction of the polymer chains. Pre-heating the solution significantly speeds up the process of removing the old paint coating, which is much safer for removing varnish from wooden components.

Now we come to the key question. What surfaces can we remove old coatings from with sodium hydroxide?

The first group is metal. Lye effectively removes varnish coatings from all types of gates, fireplaces or metal trim. However, it must be remembered that it will not remove rust. When removing coatings from wood, the key is in its type. Wood from trees such as pine (mainly used in the manufacture of doors, chairs, skirting boards, arm bands, chests of drawers and cupboards), beech (mainly chairs) or exotic sansho is usually not a problem. Simply apply a diluted lye solution to the surface of the wood and rinse thoroughly with water. This can be difficult with pine trees known as "pitch pine". This type of wood becomes fibrous when the old coating is removed, which makes repainting difficult. On the flip side, wood from elm, oak and walnut tend to discolour to a slightly darker tone when the old paint coating is removed.

The use of lye on plywood and veneer is definitely not recommended. After removing the varnish, the layers usually separate. The last group of surfaces to which chemical removal of coatings can be applied are plastics. An aqueous solution of caustic soda also works well here - use it carefully, it will not damage the polymer chains of the most commonly used plastics.

Of course, there is another group of coatings that caustic soda cannot remove. These include, for example, modern water-based primers and acrylic paints. The same applies to all oven enamels and powder coatings. To remove these types of varnish coatings, specialised chemicals are usually used, which are applied evenly to a given coating. After applying, wait for a longer period of time and then scrape off the old paint with a putty knife or scraper. In turn, the aluminium alloy begins to lose its lustre and dissolves due to the reaction with the caustic soda solution. The same effect is observed in the case of mirrors, which break on contact with sodium hydroxide. Lye solutions should not be used to remove coatings from chipboard and fibreboard. These materials are irreversibly damaged on contact with the substance.