5.4 Aircraft Finishing Systems, Registration Markings & Placards

5.4 Aircraft Finishing Systems, Registration Markings & Placards

ACS Subject AM.I.G is the second-largest General subject at 24 knowledge elements, and roughly half of them are about finishing rather than corrosion chemistry: materials used for protection of airframe structures (K14), primer materials (K15), topcoat materials (K16), surface preparation (K17), effects of ambient conditions (K18), effects of improper surface preparation (K19), regulatory requirements for replacing identification, registration markings, and placards (K20), inspection of aircraft finishes (K21), safety practices when using finishing materials (K22), application techniques (K23), and control surface balance after refinishing (K24). This section covers that half.

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1. The Finishing System Is a Stack, Not a Coat

An aircraft finish is a layered system, and each layer has a job. Skipping or substituting a layer is what causes the failures in Section 3.

+-------------------------------------------------------------------------+
|              TYPICAL METAL AIRCRAFT FINISHING STACK                      |
+-------------------------------------------------------------------------+
|  4. TOPCOAT      Polyurethane / acrylic lacquer / enamel                 |
|                  -> color, gloss, UV and chemical resistance             |
|-------------------------------------------------------------------------|
|  3. PRIMER       Epoxy, wash (etch), or zinc chromate primer             |
|                  -> adhesion bridge + corrosion inhibition               |
|-------------------------------------------------------------------------|
|  2. CONVERSION   Chemical conversion coating (chromate / Alodine) or     |
|     COATING      anodize on aluminum                                     |
|                  -> converts bare metal into a paint-receptive,          |
|                     corrosion-inhibiting film                            |
|-------------------------------------------------------------------------|
|  1. SUBSTRATE    Clean, deoxidized, dry bare metal or prepared composite |
+-------------------------------------------------------------------------+

Primer materials (K15)

• Wash primer (etching primer) — a thin two-part phosphoric-acid primer that etches and passivates in one step. It is a base for a following primer, not a standalone finish.
• Zinc chromate primer — the classic yellow-green inhibiting primer; the chromate ion actively suppresses corrosion at scratches. Its use is increasingly restricted for health and environmental reasons, so always check the current manufacturer's specification.
• Epoxy primer — a two-part primer with excellent adhesion and chemical and fluid resistance; the modern default under polyurethane.
• Urethane primer / primer-surfacer — used where a filling, sandable base is needed.

Topcoat materials (K16)

• Polyurethane — the current standard for metal aircraft: high gloss retention, excellent resistance to Skydrol, fuel, and UV. Two-part, and the isocyanate component demands supplied-air respiratory protection.
• Acrylic lacquer — fast drying, easy to spot-repair, but soft and solvent-sensitive.
• Enamel — single-stage, tough, less glossy, historically common on general aviation aircraft.
• Nitrate and butyrate dope — for fabric-covered aircraft only. Nitrate dope is used as the first coat because it adheres to fabric better; butyrate is used for subsequent coats because it is less flammable and shrinks more predictably.

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2. Surface Preparation and Application (K17, K23)

The industry maxim is that a paint job is 90% preparation. The sequence for bare aluminum is: clean and degrease, remove old finish if required, mechanically abrade or chemically deoxidize, rinse, dry, apply chemical conversion coating, rinse, dry, prime within the manufacturer's specified window, then topcoat within the primer's recoat window.

Application technique. Hold the spray gun perpendicular to the surface at the manufacturer's stated distance (commonly 6 to 10 inches), move the gun parallel to the surface at a steady rate, and overlap each pass by roughly 50%. Arcing the gun in a swinging arc puts more material at the center of the stroke and less at each end, producing banding.

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3. Ambient Conditions and Preparation Failures (K18, K19)

The exam pattern is consistent: environmental defects (blushing, orange peel) come from air and temperature, while adhesion defects (peeling, fisheye) come from contamination and preparation.

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4. Inspecting the Finish (K21)

A finish inspection looks for more than cosmetics. Report and investigate:

• Blistering — often a sign of corrosion or moisture underneath the film, not a paint defect.
• Chalking and loss of gloss — UV degradation of the topcoat, reducing its barrier value.
• Crazing and cracking — brittle film, often from excessive film thickness or age.
• Chipping around fasteners and leading edges — bare metal exposed; a corrosion initiation site that must be touched up.
• Filiform corrosion — worm-like tracks under the film, typically starting at a scratch or fastener; a specific corrosion mode driven by high humidity under an intact-looking coating.

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5. Identification, Registration Markings, and Placards (K20)

This is a regulatory element, and the numbers are testable.

Aircraft identification plate — 14 CFR § 45.11 and § 45.13. The identification data must include the builder's name, model designation, builder's serial number, type certificate number (if any), production certificate number (if any), and, for engines, the established rating. Section 45.13(b) generally prohibits removing, changing, or placing identification information on an aircraft, engine, propeller, or appliance without FAA approval — a rule aimed squarely at obscuring an aircraft's identity.

Registration markings — 14 CFR Part 45 Subpart C. The key sizes under § 45.29:

Character geometry is also specified: characters must be two-thirds as wide as they are high (except the numeral 1, which is one-sixth as wide as it is high, and the letters M and W, which may be as wide as they are high); lines forming the characters must be one-sixth as thick as the character is high; and the space between characters must be at least one-fourth of the character width.

Practical consequence for the painter: when you strip and repaint an aircraft, you must restore the marks to the correct size, spacing, and contrasting color, and you must not obliterate or relocate the identification plate. Operating limitation placards required by the Type Certificate Data Sheet, the Aircraft Flight Manual, or an STC must likewise be replaced in their original locations — 14 CFR § 91.9 prohibits operating an aircraft without the required placards and markings being in place and legible.

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6. Control Surface Balance After Refinishing (K24)

This is the finishing element with the most serious consequence. Paint is weight, and weight added aft of the hinge line changes the static balance of a control surface. An out-of-balance surface can develop flutter — a divergent aeroelastic oscillation capable of destroying the surface, and with it the aircraft, in seconds.

Rules to carry into the exam and the hangar:

1. After refinishing any control surface — aileron, elevator, rudder, tab — rebalance it to the manufacturer's specification before reinstallation, unless the manufacturer's data explicitly states rebalancing is not required.
2. Excess paint is the usual culprit. Repeated repaints without stripping stack film thickness aft of the hinge line.
3. Balance is checked with the surface removed and supported at the hinge line, with all hardware, tabs, and seals installed exactly as they fly.
4. If a surface is out of limits, the correction is made with the balance weight specified by the manufacturer — never with an improvised weight or by removing structure.
5. Safety when finishing (K22): finishing materials are flammable and toxic. Spray only in an approved, properly ventilated booth with explosion-proof lighting, bond and ground the spray equipment and the aircraft, and use the personal protective equipment the Safety Data Sheet specifies. Two-part polyurethanes containing isocyanates require supplied-air respiratory protection — a cartridge respirator is not adequate.