Craters or silicones are one of the most common defects in the repainting process. Almost certainly, most professional painters have had to deal with this painting defect at some point.

With this tutorial you will learn why they are formed and how to solve this problem.

What are silicones or craters in paint?

Silicones are defects in car refinishing that appear in intermediate layers and above all in the finish, giving rise to circular cavities that are very similar to the shape of a crater. In some cases, impurities can also be observed in the centre of the crater.

Superficie pintada con siliconas y crateres

The appearance may be different depending on where the silicones are located, with flatter cavities in the top layer of the paint, up to severe moisture problems that penetrate to the substrate. 

The main cause of their appearance is due to the lack of moisture of the substrate or base layer, leaving these visible, to a greater or lesser extent, depending on the surface tension.

They are usually produced due to the presence of contaminants in the paint such as silicones, greases, waxes or oils that repel paint. These contaminants can be found in the material used to clean the part (cloths), or in tools used during the painting process, such as a spray gun or the compressed air line.

The size of this defect can vary, ranging in size between 0.5 and 5 mm, which influences the subsequent repair process. 

This is important because if the cracks have not been completely repaired and removed,  the craters can become visible again in the form of flat cavities.

Causes for the appearance of silicones

The main causes of the appearance of craters or silicones have to do with the presence of contaminants in different supports, from people and tools, to paint booths and materials.

Inappropriate cleaning of the surface

The use of dirty cloths, containing residues of resins or other substances, added to a non-uniform cleaning movement of the support, will contribute to the formation of craters.

Environmental pollution, mainly by silicones

The presence of silicone can occur both in the environment and on the support. This problem can occur even in cases where the exposure of the part to silicones has been for a short period of time.

Presence of water or oil in the compressed air

You should always ensure a correct cleaning and maintenance of the compressed air line, preventing it from containing traces of water, grease or oil.

Dirt on application equipment

The presence of contaminants such as grease, soap, detergent or oil in work tools such as spray guns or hoses should be avoided, always ensuring that they are in an optimum clean state for work.

Lack of maintenance of the spray booth

Booth smoke saturation caused by combustion chamber leaks, and dirt on roof filters and floors, and sealing agents are possible causes of this defect.

Measures to prevent the appearance of silicones

Correct cleaning of the part or support

Meticulously clean the part you are going to work on with a suitable degreaser. Do not let the cleaning solvents dry on the surface: remove them with a clean, dry cloth and change this cloth with each cleaning. It is advisable to completely remove any silicone polish from the surface to be painted and avoid using this type of polish near the paint section.

Spray booth and compressed air line maintenance

Control the status and functioning of the booth and air-line, renewing the filters, cleaning the ducts and carrying out periodic purges of compressed air in the facility.

Cleaning of painting tools

All items involved in the application process must be correctly decontaminated from any type of substance. Something as simple as an impurity in a mixing rod can give rise to this problem. In addition, always use a suitable solvent for cleaning the spray gun and dry it thoroughly, especially when using water-based products.

Steps to solve craters in the paint

To solve the appearance of craters or silicones in the paint, you must first allow the finish to dry completely, in order to correctly assess the severity of the problem. 

  1. If the craters are not very large, a good solution would be to sand the surface of the part with P1500 sandpaper, and subsequently polish and glaze the surface.
  2. If the defect is deep, you should sand down to a surface that is free of defects. Then clean it meticulously with anti-silicone solvent and start the painting process from the base.

In specific cases, and if the defect is very noticeable, we can use a special anti-silicone additive, always following the indications provided on the technical data sheet.