Painting a car
Important information about paint in the classic car and automotive sector
The world of colours and thus of paintwork in the automotive sector is fascinating and covers many topics, which we would like to outline here.
A phenomenon and an almost daily topic is colour rendering and colour measurement, the painting of adjacent components, the angle of incidence / angle of reflection of light and related problems and issues.
1 Lacquer types
If a classic is to be painted as part of a restoration project, modern paints are the preferred choice. The reasons for this are obvious: they are easily accessible, adapted to modern painting techniques, inexpensive, durable and easy to apply. Even though restorers could resort to old lacquers such as linseed oil, nitro or alkyd resin lacquers for reasons of originality, this often does not happen, because on the one hand their processing is very labour-intensive and requires a lot of expertise, and on the other hand the old lacquers fall under the Ordinance on the Limitation of Emissions of Volatile Organic Compounds (in short: ChemVOCFarbV).
This rule is also confirmed by an exception: Under § 3, paragraph 3b, the ChemVOCFarbV permits the sale, purchase, preparation and application of limited quantities of old paints for vintage cars that are classified as being of special historical and cultural value. However, permission must be obtained from the competent authority for each individual case. Link to the site:
http:// www.gesetze-im-internet.de/bundesrecht/chemvocfarbv/gesamt.pdf
Nitrocellulose paints, for example, may still be used, even if they are only offered by very few manufacturers and in small quantities (see info box), in compliance with the 31st BimSchV (Ordinance on the Implementation of the Federal Immission Control Act). For paint shops, it is therefore important to involve the owner and a specialised expert in the decision whether to work with historically correct or modern paint materials for a vehicle of particular historical and cultural value - this can also be an early VW Beetle or an EMW 327 convertible, depending on the interpretation and the expertise of the authorities. Only a specialised expert can assess whether or not the use of modern paint materials results in a reduction in value.
1.1 Linseed oil copal varnishes until approx. 1910
Until about 1910, the body panels were still painted with linseed oil copal varnish. The top coat consisted of a binder, a mixture of linseed oil and natural resin, and pigments for colouring, mostly carbon black. Black was therefore the most commonly used colour. All other colours were very expensive. Painting was done with a brush, the paint was thinned with alcohol. Impregnating agents for the wood did not yet exist. The addition of alcohol made the painter's work very dangerous because of the explosive vapours that were produced.
The biggest problem, however, was the long drying times and the sensitivity of oil paints to dust. Thus, the curing of the paint coat took up to 8 weeks.
[1] Excerpt from the book "Taschenbuch für die Farben- und Lackindustrie" by Erich Stock (1940)
Oil varnishes based on linseed oil or, if special quality was required, amber varnishes, which were obtained from liquefied amber resins and were very expensive.
The term "amber varnishes" was still used for a long time, even when synthetic resins were gradually replacing natural resins. The painter originally produced his paints himself, first manually, then with hand-operated paint mills.
This was done as painters had done centuries before, by rubbing binder and pigments together on sandstone or marble slabs with a so-called runner. Since the paints were produced anew each time by mixing the pigments with the binder, the colour shade often remained a random product. The buyer could choose whether his car was to be blue, black, green, maroon, beige or red - but it was not possible to choose an exact shade. The colours all had a relatively earthy tone, because there were only mineral colours that contained inorganic pigments. Lead white and lead red lead were mainly used as anti-corrosion pigments.
In the early days of car manufacturing, there were the following surfaces to work on: the chassis, the bonnet and the body. Before painting, the surfaces had to be laboriously smoothed, as the sheet metal parts were driven by hand or with a mechanical hammer. Between four and eight weeks passed before a car was completely painted. Several coats of putty and some intermediate coats with generous drying time were necessary. In addition, there was a simple, faster-drying varnish based on wood oil that required "only" about ten working days.
1.1 Shellac based on animal resins from 1910 to 1920
From 1910 to 1920, shellac based on animal resins was used. This required a drying time of four to eight weeks, which prevented series production on the assembly line. It was only later that the drying time for car bodies could be reduced to about half by using heated drying rooms and shellac mixed with methylated spirit.
Properties:
1.2 Nitrocellulose base as of 1920
From 1920 onwards, the newly developed nitrocellulose-based lacquer, cellulose nitrate lacquer, or nitro lacquer for short, was used sporadically, and then increasingly from 1925 onwards.
However, it was again the Americans who made the start. In 1923, General Motors and Ford painted their cars with the new dye.
In 1912, Henry Ford had an automobile produced on the assembly line for the first time - the Model T. Subsequently, this production method changed the entire sequence of (assembly line) production. Ford soon realised that the painting process was a stumbling block, because the speed of the paint shop could not keep up with the pace of the assembly line. Ford therefore looked for a way to speed up the painting process. But it was not until after the First World War that researchers discovered a new material for paint production: nitrocellulose.
Chemically, the new car paint is based on a raw material that was used as gunpowder in World War I - nitrocellulose. Nitrocellulose could be processed into paint binders. In addition, chemists were now able to produce plasticisers, some solvents and synthetic pigments on an industrial scale. Cellulose nitrate is produced in the chemical industry by reacting cellulose with nitrating acid. Formally, this is the reaction of an alcohol with an acid to form an ester. The nitrogen content of the cellulose nitrate to be produced is controlled by the composition of the nitrating acid and the reaction time. If the nitrogen content is > 12.75 %, it is predominantly cellulose trinitrate (gun cotton), and if the content is < 12.75 %, it is cellulose dinitrate (collodion wool).[3]
The spray gun evolved from a compressed air atomiser developed by an American country doctor for spraying the throat with antiseptic agents.
[1] Excerpt from the book "Taschenbuch für die Farben- und Lackindustrie" by Erich Stock (1940)
[2] Excerpt from the book "Taschenbuch für die Farben- und Lackindustrie" by Erich Stock (1940)
[3] Excerpt from "Handbuch der Nitrocelluloselacke" Part 2 by Dr. Alfred Kraus (1952)
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