Tension varnish

Tension varnish is a special nitrocellulose varnish that is used in aircraft, model aircraft and airship construction to impregnate and tighten fabric (covering).

history

In the early days of aviation, various attempts were made to provide the basic structures of the flying machines, mostly made of wood, with a fabric cover that was as weatherproof, airtight as possible and hard-wearing. A lacquer made of celluloid or cellon dissolved in acetone soon proved to be the most suitable . The process of soaking and painting the aircraft several times was also known as "celloning".

process

After the wooden structures have been sanded down, cleaned and possibly impregnated against fungal attack, a so-called adhesive varnish (usually a thickened stretch varnish) is applied to all the places where the fabric should later adhere. Once this has dried, the fabric is applied with as little wrinkle as possible and the adhesive varnish is activated through the fabric by soaking the areas with highly diluted tension varnish. In this way, the fabric is fixed and further work can be carried out, such as attaching narrow fabric tapes to areas that will later be heavily used or sewing the fabric to the framework (using a circular needle ). The latter is especially necessary in concave areas such as the underside of the wing, as the later elasticity of the paint can detach the covering there again. The entire construction is then painted several times with diluted stretch varnish until the fabric is soaked and filled to the point that it becomes impermeable to air. The tensioning varnish thus forms the matrix of the covering and ensures complete bonding with the basic structure of the components. The property that gives the tensioning varnish its name is the shrinkage when the solvent evaporates. Thus, the covering is tightened evenly and forms a relatively hard-wearing surface comparable to a fabric film. The elasticity of the lacquer can be partially influenced by adding various oils (e.g. castor). The big advantage is that damage to the covering can be repaired relatively easily by simply cutting out a field and then sticking a new piece of fabric with tension varnish. Components such as B. Plywood rings as reinforcement for rope ducts can simply be applied with tensioning varnish and connected to the fabric by repainting them several times. Since nitrocellulose lacquers are generally hygroscopic, a top coat of paint with a (possibly colored) water-repellent top coat is recommended.

Airship construction

The outer hulls of the rigid airships were probably the largest uses of tension varnish in history. Here, too, the cotton covering was stretched and impregnated by painting it several times. This was followed by an outer layer of paint with pigments or aluminum powder. The last two Zeppelin airships LZ 129 and LZ 130 each had around 34,000 m² of fabric to be sealed.

General aircraft construction

Until the emergence of sheet-covered all-metal aircraft, the open spar-rib construction and half-timbered construction with fabric covering were standard in aircraft construction until the mid-1930s. The first modern low-wing fighter planes such as B. the Hawker Hurricane in their first model series were still largely fabric-covered. But even during the Second World War, fabric covering was retained on many components, primarily on control flaps (e.g. on the Me 163 ).

Glider construction

Until the advent of plastic gliders, the fabric-covered construction was common in glider construction and is still widely used today. The fabrics originally used were cotton, silk or, later, diols . Little by little, self-adhesive fabric films are being used today, which shrink under the influence of heat and make stretch varnishing superfluous.

Modelling

Here the process is similar to the one previously described. However, the covering (covering paper , silk, rayon) is first applied with simple paste and, after the paste has dried, it is connected to the overall structure with the stretch varnish. After the paste has dried and before the actual varnishing, the covering paper is once again slightly moistened, whereupon it tightens and wrinkles disappear after drying. Here too, complete drying must have taken place before painting, otherwise the otherwise transparent stretch varnish will turn white and streaky. In model making, a top coat is usually not used, instead pre-colored types of tensioning varnish are available. For drying lightweight components have a slipway are clamped (jig) because the strong tensile forces can cause warping of the entire structure during drying of the paint. For the same reason, the corresponding upper and lower sides of a component must be painted at the same time.

dilution

The so-called nitro thinner or solution is used for thinning, dissolving or cleaning brushes . It usually consists of esters and other hydrocarbons . Alternatively, universal thinners can also be used, but it can happen that oily components in universal thinners impair or cancel the elasticity.

hazards

Clamping lacquer contains a lot of solvents. So processing should only be done with the best ventilation. All known solvents are easily flammable and form flammable vapors at the appropriate concentration. Even when dry, stretch varnishes are often very flammable (similar to celluloid ).