This chapter covers the principles, materials, and procedures for painting and finishing aircraft structures. Proper painting is not merely cosmetic; it provides essential corrosion protection, UV resistance, and aerodynamic smoothness. The chapter details surface preparation, paint types, application techniques, defect identification, and safety precautions critical for aircraft maintenance engineers.
Key Concepts Explained
1. Surface Preparation
Surface preparation is the most critical step in any painting process. Proper preparation ensures adhesion, prevents corrosion, and determines the final finish quality.
Cleaning Sequence:
Wash with soap and water to remove oils, grease, and dirt
Rinse thoroughly to remove all soap residue
Use a tack cloth immediately before painting to remove dust and lint
Avoid solvent wiping as it may leave residue that affects adhesion
Surface Treatment for Different Materials:
Paint Removal:
Chemical paint stripper is preferred for aluminum skins to avoid metal damage
Sandblasting risks surface damage and warping
Aggressive sanding (e.g., 80-grit) is too harsh
Heat guns can damage aluminum structure or adjacent components
2. Paint Types and Their Applications
Polyurethane Paint (Exterior Finish)
Most common for aircraft exteriors
Excellent durability, UV resistance, and gloss retention
Two-part system: base + hardener
Requires induction time (pot life) after mixing before application
Must be stored unmixed in a cool, dry place
Primer Types:
Etch primer (wash primer): For aluminum; contains acids for chemical etching
Epoxy primer: For composites; provides adhesion and flexibility
Zinc chromate primer: For aluminum corrosion protection
Clear coat: Applied over color coat for UV protection, gloss enhancement, and scratch resistance
Special Considerations:
Radomes require non-metallic paint to avoid radar signal interference
Stencil paint must be compatible with existing paint, provide good contrast, and be fade-resistant
3. Application Techniques
Spray Painting Procedure:
43.Mix base and hardener according to manufacturer specifications
44.Allow induction time for chemical reaction to begin
45.Apply in thin, even coats
46.Maintain correct gun distance and air pressure
Repair Sequence for Paint Chips:
49.Clean area to remove contaminants
50.Sand edges to feather (taper) the repair area
51.Apply primer
52.Apply topcoat
Feathering: Sanding the edges of a repair area to a thin taper. This blends new paint with old, removes rough edges, and improves adhesion by providing a smooth transition.
4. Paint Defects and Causes
5. Weight and Balance Considerations
Painting adds weight to aircraft components. For control surfaces:
Re-balance the surface after painting to meet specifications
Lightweight paint and thin coats help but do not eliminate the need for re-balancing
Avoid painting trailing edges is not standard practice
Important Procedures and Regulations
Safety Precautions (Most Critical)
When painting in a hangar:
68.Adequate ventilation – Prevents fume buildup (paint fumes are flammable and toxic)
69.Respirator – Protects against inhaling harmful vapors
70.Fire extinguishers – Necessary due to fire hazard from flammable paints and solvents
Storage Requirements for Two-Part Paints
Store unmixed in a cool, dry place
Once mixed, paint has limited pot life and cannot be stored
Do not refrigerate
Protect from sunlight
Inspection Checklist
Before painting:
[ ] Surface clean and free of contaminants
[ ] Old paint properly removed (if required)
[ ] Surface treatment applied per material type
[ ] Primer compatible with topcoat
[ ] Tack cloth used immediately before painting
After painting:
[ ] Check for defects (crazing, runs, orange peel, blisters)
[ ] Verify cure time and conditions
[ ] Re-balance control surfaces if applicable
Relationships Between Concepts
Adhesion Chain: Surface preparation → Primer application → Topcoat application → Final finish quality. A failure at any step compromises the entire system.
Material-Specific Requirements:
Aluminum requires etch primer for adhesion
Magnesium requires chromate conversion coating for corrosion protection
Composites require flexible epoxy primer
Fiberglass radomes require non-metallic paint
Defect Causality:
Incompatible layers → Crazing
Excessive thickness → Runs/sags
Improper technique → Orange peel
Trapped moisture → Blisters
Contamination → Poor adhesion
Safety Interdependence: Ventilation + respirator + fire extinguisher form a complete safety system. No single measure is sufficient when working with flammable, toxic paints.
