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Builders guide to safe aircraft materials
Surface coatings and finishes
Rev. 3 — page content was last changed 25 January 2010
Airframe structures are coated internally and externally to provide protection from corrosion effects, and from deterioration caused by solar UV radiation, biological attack or trapped moisture. The latter not only because of deterioration/corrosion of metal structures or biological attack or swelling/shrinking of wood structures, but also because water seeping into control surface interiors can lead to control flutter. Internal ventilation plus properly applied and maintained paint is one of the main forms of corrosion inhibition, the other is plating steel components with a sacrificial metal.
In addition, various fillings and coatings are applied to the external surface contours to promote a non-turbulent airflow that remains attached to the skin. Final colour finishes are applied for both protection and aesthetic reasons. Regular polishing of some finishes will be necessary for maintenance of the finish. Some owners of aluminium-skinned aircraft polish and seal the bare external metal in preference to painting.
In some types of airframe construction the amount of labour-intensive surface preparation necessary before and during the application of the various coatings comprises a considerable part of the total construction task; in other types there may be little need for surface preparation and coating. synthetic resin providing sealing and adhesion; the pigment, which is usually finely ground solids held in suspension in the liquid adding opacity and colour; and the solvent, which dissolves the binder/resin and holds it in solution while the mixed liquid is applied by brush, roller or spray. Some resins are soluble in water or may be suspended in it; where water is the vehicle, the paints are referred to as water-borne rather than solvent-borne.
After application a chemical reaction occurs while the solvent/water evaporates, leaving the resin as a solid film that binds the pigment particles together. Some pigments add excellent ultraviolet [UV] insolation protection, particularly aluminium flakes in powder or paste form. In addition to the main components, small amounts of particular purpose additives are included in the paint formulation either at the factory or by the user at the time of application. For example:
The latent heat for vaporisation is drawn from the surface that the solvent is touching, so those surfaces will be cooled; the degree of cooling is dependent on the rate of evaporation which, for a particular solvent, is affected by temperature. Atmospheric water vapour will condense on the cooled area (the degree dependent on relative humidity) and, if retained, will cause white or cloudy areas in the paint film — blushing — which will be difficult to avoid if working in conditions of high temperature and high relative humidity.
Solvents are expensive, so cheaper and less volatile diluents — thinners or reducers — may be added to the formulation during manufacture to lower the viscosity of the paint or, when appropriate to the intended application or method, may be added by the user. (The paint formulation supplied by the manufacturer is usually 'thicker' than generally required, because it is much easier for the user to thin a 'thick' mix than it is to thicken a 'thin' one to get the viscosity appropriate to the atmospheric conditions, the job and the method.) Diluents do not dissolve the binder/resins; rather they promote the ability of the solvent to absorb more resin (and perhaps accelerate the cure time a little). There may be a stage where excessive addition of diluent causes the resin to precipitate from the solution, thus rendering the paint useless.
After initial cleaning, the surface energy of aluminium substrates is usually increased by light abrasion perhaps with Scotchbrite pads (constructed from nylon fibres coated with very fine aluminium oxide grit) and an acid etching/cleaning solution such as Alumiprep 33. This is done so that metal oxides are removed, some of the surface molecular bonds are temporarily broken and, rather than being dead smooth, the surface is roughened to provide a greater attachment or 'keying' area. The surface preparation should not be started unless all machining (hole drilling, countersinking, edge chamfering, bending) is completed.
All substrates must have sufficient surface energy to overcome the natural surface tension of the liquid solution being applied to it, thus encouraging the liquid to spread evenly and adhere to the substrate. The lower the surface tension of the coating liquid the higher its wetting ability.
(Break-out or water break free test: if pure water is placed on a low surface energy substrate — a newly waxed and polished car is a good example — the water will not spread out evenly but will form into globules and rivulets. The test for acceptable surface energy after cleaning and prior to initial coating is to dribble pure water onto the surface. If it spreads in an unbroken film, the surface is ready. Mylar laminated fabrics used for aircraft wing covers or leading edges on trike wings have very low surface energy; rather than sheeting over the surface rain will form into globules and rivulets, tripping the intended laminar boundary layer flow at the leading edge. This decreases wing performance, unless a surfactant, such as a kitchen detergent mixture, is wiped over the leading edge.)
Chemical conversion coatings for aluminium alloys react with the substrate to accelerate the formation of the passive oxide barrier integral to the metal. This improves the adhesion of subsequent primer/ paint coatings and corrosion resistance. Chromic acid-based coating chemicals may change the substrate colour, Alodine 1201 for example produces a light gold chromate conversion coating when applied to aluminium alloys, which is a useful means of checking for a consistent result. If the colour tends towards brown, the conversion coating may be excessive and perhaps inhibit adhesion of the next coating. Of course after all machining is completed airframe components could be sent to a commercial facility for anodising. This would give the best result, but the process is expensive and probably impractical except for a few major structural parts.
The phosphoric acid-based conversion coating system usually applied to chrome-molybdenum steel in aircraft structures converts to a zinc phosphate crystalline layer which chemically bonds to and passivates the metal so that corrosion is inhibited. Zinc phosphate also provides a suitable foundation for adhesion of primer, paint or powder coatings.
The metal conversion coatings are applied by brush, wiping pad, spray or — for smaller components — dipping in the solution.
There are quite a number of proprietary conversion coating solutions available and the instructions regarding pre-cleaning, acid etching solutions, conversion coating, application times, rinsing and drying, plus compatible primers and top coats, must be followed otherwise the entire surface finish could be an expensive failure.
Zinc chromate primers (green or yellow in colour) are applied over conversion coatings on steel or aluminium alloy substrates, but are best used as a stand-alone interior anti-corrosion coating rather than as a base for further coating. Topcoats add weight, are costly, entail a lot of work, are unnecessary for enclosed interiors and may mask corrosion development. Zinc chromate should not be used under a polyurethane top coat. Zinc chromate has been in use since at least the 1940s, and is the yardstick by which modern primers are measured. Epoxy strontium chromate primers are commonly used in the aircraft industry. All chromates are thought to be carcinogenic so appropriate precautions must be taken.
Some types of phosphoric acid-based two-pack primers are designed to be applied as a very thin wash coat to penetrate and fill any microscopic pits in the substrate before applying a normal viscosity primer.
Two-pack epoxy primers are very durable and suitable for steel, aluminium alloy, wood or composite substrates. They are commonly used under urethane/polyurethane, acrylic lacquer and most other top-coats. Two-pack epoxy primers are generally applied as a low-viscosity first coat followed by two normal-viscosity coats. There will be an optimum minimum/maximum time period between application of the primer and the application of the topcoat. Most two-pack epoxy primers are not easy to sand and may be used as a stand-alone interior protective coating. Most other primers are formulated with the expectation of top coats being applied to complete surface sealing. Two-pack urethane primers are particularly formulated for use under urethane/polyurethane finishes. Epoxy chromate primers are suitable for metal and composites.
Note: two pack products are sometimes referred to as '2K'.
Faying metal surfaces should be conversion-coated and primed before riveting/assembly, and the joint edges or gaps sealed with a sealant to inhibit crevice or galvanic corrosion.
Varnish is a generally un-pigmented binder-solvent solution applied to protect internal wood surfaces with a transparent film that dries by chemical reaction on exposure to air. The most popular airframe varnishes are two-pack epoxies applied directly to the wood/plywood structure as a thinned sealing coat, followed by two normal viscosity coats. Epoxy varnish is not affected by the chemicals contained in fabric cements and can be applied over the airframe following epoxy adhesive bonding, but it takes about a week to fully cure. Spar varnish is a high-quality waterproof and sunlight-resistant varnish designed for marine use and built up in many thin layers. The true spar varnish, though, is made from natural oils (tung oil for example) and there is a very long drying time (around 7–10 days) between coats. Fabric cements will lift spar varnish and possibly some of the polyurethane varnishes, though cured polyurethane varnishes are not soluble in the original solvent.
Dopes and coating systems for fabric-covered aircraft have been outlined in the module 'Aircraft fabric covering systems'.
Acrylic lacquers are fast-drying synthetic resin paints for metal or wood, pigmented for colour and which dry to a satin finish; they are generally rubbed with a compound or polish for a higher gloss. Lacquers form by solvent evaporation only. The dried coating can be readily dissolved with the original solvent while enamels undergo a chemical change (cure) after solvent evaporation. Acrylic lacquer finishes may bleach if overexposed to the sun.
Acrylic enamels are adapted from the old oil-based synthetic enamels by the addition of acrylic resin where curing can be affected by absorption of atmospheric oxygen. But it takes a long time so a catalyst is usually added to accelerate the curing time to a few days, providing a hard, tough smooth surface with medium to high gloss. These are not affected by solvent once cured.
(Enamel is a generic term for glossy topcoat paints, in particular those that form cross-linked bonds during curing.)
Acrylic polyurethane enamels are available in many formulations including single-pack pre-reacted polyurethane and two-pack catalyst/polyurethane topcoats. DuPont's Imron is a tough, durable and flexible two-pack acrylic polyurethane enamel developed for car refinishers. Polyester polyurethane enamels are generally more expensive, but tougher and more resistant to chemicals. Pigmented polyurethanes have a high degree of UV resistance, which can be reinforced with a UV inhibitor and provide a deep, high-gloss finish. (Manufacturers may toss in a few adjectives such as 'aliphatic', 'linear', and 'isocynate' when referring to their two-pack acrylic polyurethane or polyester polyurethane products; but the properties referred to are generally common to both types. Polyurethanes are also described as 'urethanes'.)
Note that the polyurethane enamels formulated for use on metal or composite substrates are not flexible enough to topcoat fabric skins; they require a plasticiser additive.
Some two-pack polyurethanes are applied as a single-stage 'topcoat' system while others are applied as a 'basecoat/clearcoat' system — pigmented flat 'base' coatings finished with high gloss 'clearcoat'. The latter provides the lustre and 'wet-look' sheen. Thet are more likely to be used for trim/accent colours rather than the main surface coating because the clearcoat adds weight, with the only return being the sheen .
Polyurethanes may be applied as an initial 'tack' coat followed by one or two coats as needed to attain the desired finish; a thick coat may crack. After curing, 'topcoats' are fine-sanded by hand to obtain a very smooth surface, then polished (but not waxed) for a 'perfect' finish.
There is some difference in the thickness of the paint coat as applied. Polyurethane enamels formulated for automotive use (where weight is not a particular problem) tend to be thicker, and thus heavier, than enamels specifically formulated for aircraft. There are various pigment powders marketed that provide metallic and other finishes; for example titanium dioxide-coated mica platelets (pearls) which provide apparent depth and diffraction effects — pearlescence.
Computer-generated colour accents. Rather than painting trim colours the use of computer-generated vinyl trim and graphics saves a lot of masking and preparatory work. This requires cooperation between the builder and a commercial supplier to select designs and colours that suit the aircraft before the trim [(and registration number) decals are delivered.
solar radiation is in the ultraviolet range, 40% in the visible light range and 49% in the infrared range. It is the infrared radiation, both as direct and reflected radiation, which contributes mostly to the heating of a parked aircraft exposed to clear sky insolation. The temperatures that can develop within the airframe structure are quite high and are, to a large extent, dependent on the colour of the aircraft skin. Quite a few years ago the Australian Department of Civil Aviation released a document that included temperature readings on fibreglass-reinforced polymer panels with various colour surface finishes. The ambient atmospheric temperature was a high summer 40° C. The following table records the surface temperatures of the panels.
Heat soaking can have a deleterious effect on composite structures; the problem is related to the glass transition temperature and is discussed in the post-curing section of the module 'Plastics and thermosets'. White is the only suitable overall finish for composite aircraft and care must be taken in the choice of trim colours to minimise differential heating of the structure.
Heat soaking also affects wooden aircraft. The strength of wood is inversely proportional to the temperature — about a 1% reduction in the ultimate strength and stiffness values for each 1 °C increase in wood temperature. So if the temperature of the internal structure is increased from 20 °C to 50 °C, the strength is reduced by 30%. Short-term heat soaking will not permanently affect the strength of wood structures but long periods at high temperatures will reduce the ultimate modulus of rupture and modulus of elasticity values. See effects of heat in the module 'Properties of wood'.
Heat soaking problems are not confined to composite or wood aircraft structures. Aluminium aircraft can also be affected as Lynn Jarvis found when he chose a deep blue pearlescent polyurethane paint scheme for the wings of his Sonex project.
Screening pigments and metal powders, such as titanium dioxide or aluminium flakes, reflect UV radiation. In addition there are two types of stabilising or UV-inhibiting additives in general use:
Polishes are often polymer-based but, unlike waxes, they are formulated to fill pinholes and microscopic imperfections in paint and polyester gel finishes, thus sealing them from moisture and contaminant ingress. Both waxes and polishes provide the gloss and protect the surface, but the polishes perform better because of their sealant properties.
Bare metal polishes contain varying grades of abrasive particles, for heavy abrading down to fine finishing. These rubbing or polishing compounds remove the surface corrosion and fine scratches from bare aluminium to leave a mirror-like finish on the bare metal, which is usually maintained by application of a wax or sealer.
Hardware fittings should be either stainless steel or chromium-molybdenum steel; the former requires little or no corrosion protection, the latter will need treatment — as will chromium-molybdenum undercarriage, engine mount and similar structures. Corrosion-inhibiting compounds could be used between dissimilar metals or alloys. Bolts, nuts and other steel fasteners must be cadmium or zinc plated. (See the next section for treatment of chromium-molybdenum steel.)
Sailcloth skins require no surface treatment other than protection against deterioration from UV radiation by some form of UV-blocking agent applied to the fabric. There are liquid blockers (303 Aerospace Protectant for example) which should be applied perhaps several times per year.
Colour schemes for tube and fabric aircraft are accomplished by fabricating cover segments from sailcloth dyed in different colours. When fibres are dyed the colour solution enters the internal structure of the fibre. Whereas when fibres are painted the pigment is bound onto the surface of the fibre. Thus when the colour of dyed sailcloth deteriorates — and is not recovered by cleaning — the fibres are also deteriorating. Aircraft fabric covering systems' so it is only necessary to look at the initial preparation and coating of chromium-molybdenum steel tubing and wood airframes.
Chromium-molybdenum airframes: all rust and contamination must be removed completely from the inside and outside steel surfaces — either by gentle abrasion with fine aluminium oxide grit or by bead blasting (sand blasting is unsuitable for the thin-walled tube) — followed by immediate application of two coats of epoxy primer. Alternatively, for a superior finish the welded frame could be dry epoxy coated at a commercial powder coating and heat reflow facility.
For an internal rust-inhibiting coating it is advisable to slosh boiled linseed oil around the dry interior of all tubing (and particularly the critical structural components such as longerons, engine mounts or 'carry-through' tubes) before sealing. (The boiled linseed oil dries very much faster than the raw oil.)
Wood airframes: wood and plywood must be sealed to:
• maintain the moisture content at a 'normal' level
• minimise swelling and shrinking due to changes in atmospheric humidity
• prevent fungal, micro-organism and insect attack; and other deterioration problems due to contact with moisture
• prevent weathering.
Gluelines must also be protected from attack by fungi, other micro-organisms, fuel, oil and chemicals.
Epoxy varnish, after filling holes/depressions with an epoxy type filler and sanding, is the usual sealing method. Wood and plywood surfaces are normally protected with a light fabric covering so the adhesive and overlay coatings are dependent on the fabric coating method chosen; see the chemically bonded fabric covering process.
Although composite structures do not suffer from corrosion it is important to provide an impermeable surface coating to keep moisture out of the laminate, which will be weakened — even to the extent of delamination — by the freezing and thawing of trapped moisture.
Surface preparation involves cleaning, rough sanding the epoxy back to a reasonable surface, forcing a lightweight micro:epoxy filler mix into the exposed weave then sanding. The filling and sanding is repeated until a satisfactory surface is achieved then finish off with a UV-blocking primer/surfacer to fill all pinholes and provide UV protection. After final sanding, the required base for the topcoat (probably single-stage two-pack polyurethane) is achieved. After application the top-coat is buffed to achieve the finish required.
Builders guide to aircraft materials – fabrics, composites and coatings modules
| Guide contents | Aircraft fabric covering systems | Plastics and thermosets |
| Reinforcing fibres and composites | [Surface coatings and finishes] |
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