The New Zealand Railways Magazine, Volume 1, Issue 5 (September 24, 1926)
Paint and Duco
The term Colour as applied in painting always refers to any mixture that produces a separate hue.
Colours are divided into three classes; primary, secondary and neutral. The primary colours are blue, yellow, and red. By a mixture of any two of these a secondary colour is produced, and by the addition of white and black in combination with primary or secondary colours, all neutral colours, shades and tints are produced.
A colour is neutral when two colours composing it are so evenly divided in strength that one does not dominate the other, such as: yellow-green, blue-green, etc. There are also warm and cold colours. When colours are in harmony with each other they partake of the same general effect, such as chrome yellow and sienna, chrome yellow and amber, or colours or tints of red or yellow. These are termed warm colours. Cold colours are such as gray, lead, green, blue, etc.
Raw linseed oil is the produce of oil pressed from flax seed; it is not used a great deal for ear work, but is used in large quantities by house painters. Boiled linseed oil is prepared by adding litharge, umber and red lead to raw oil. First the clear oil is heated to 200° F. the ingredients added and then heated to 400° F. After it has cooled it is strained.
Turpentine is not a drier, but a thinner, and is the oil that exudes from the Terebinth tree.
Varnish is a solution of various gums made by boiling together in turpentine, until a thick transparent liquid is secured. The difference in hard, rubbing and finishing varnishes is made by the different mixing of gums.
Driers.—The principal of these are red, white and sugar of lead, litharge, oxide of manganese, gold size and terebine. The latter two are in most demand. In most of the big paint shops in Britain you will see a card telling how to mix different shades and colours, but on the Railways in New Zealand where only two or three colours are used an apprentice does not have the opportunity of learning colour mixing. The painting of cars by brush does not require any mentioning, but I will endeavour to enlighten some of the readers on dueo and air pressure painting.
A chemical director of one of the largest paint shops in America set out to produce a paint that could be applied quickly and that would dry rapidly. He wanted one with all the old advantages yet with none of the disadvantages. One that would stay permanently and wear longer without loss of lustre or protecting power. It also had to be harder and tougher so that it would not scratch or crack, and be proof against water, oil, grease, and the actions of acid or whatever preparation is used in cleaning. After four years of experimenting and severe testing, the director claimed he had produced the paint which he had set out to compound. He named this nitro-cellulose, commonly known in New Zealand as duco. It is a mixture of cellulose, butzol alcohol and nitrate cotton. Before the pigments are applied the paint is transparent (known as lacquer) and is used for inside work. It can be stained to whatever shade is required and three coats can be applied in one hour. For outside work there are approximately 70 shades to choose from. There are also filling and priming coats. These are applied by means of the spray gun and include filling and priming. Each coat takes from 15 to 20 minutes to dry. Any vehicle made ready in the morning to receive duco could be finished in one day and would be ready for use immediately.
In my opinion duco would not be a success if applied over varnish or paint as one is quick drying and the other slow. Both have different chemical actions and this would most likely cause cracking; but when applied to bare material (wood or iron) it has proved successful.
A car finished in this way will not show the page 47 same high gloss as varnish gives, but after exposure for a few months the degree of brilliancy will be equal to that of a varnished ear, as it does not become “flat” through cleaning and weather conditions.
Duco is easier and more simple to apply than brush paint. It is put on by means of a spray gun which is worked by air pressure. Each machine has an air transformer fixed to it. This removes all oil, water and dirt from the air, thus ensuring a clean job. The spray method, though successful when using duco material, is not recommended for using brush paint on car surfaces.
Many private shops in New Zealand have this process installed and it is giving entire satisfaction. If ever the larger shops adopt it adequate ventilation should be insisted on, and if each operator wears a respirator there would be little likelihood of anyone suffering ill effects through the use of the spray.
In Britain the railway cars are being sprayed with duco and it has proved successful. America, France and Italy have also adopted it. As it produces a better job in a third of the time, it is not likely to be long before, as far as coach painting is concerned, the spray will take the place of the brush.