Inspecting, treating, preparing

Seven procedures. Procedure 0 runs before every one of them, because everything in this unit involves a chemical.

Procedure 0 — Establish the chemical and the material (R3, R4)

1. Identify the material you are about to work on — aluminum alloy, magnesium, steel, composite, acrylic, painted or bare (GEN-E Concepts §5).
2. Identify the product you intend to use, by its actual name and part number.
3. Read its safety data sheet: PPE, ventilation, incompatibilities, spill response, disposal.
4. Confirm the product is approved for that material by the aircraft manufacturer.
5. Set up ventilation, containment and waste collection before opening anything.

Check — name the material and name the product. If either is "some kind of cleaner" or "aluminum probably", stop. The wrong cleaner on the wrong material causes the damage you were trying to prevent (GEN-G Concepts §4).

Failure — a strong alkaline cleaner on aluminum, or the wrong solvent on acrylic. Both look like cleaning at the time.

Procedure 1 — Perform a corrosion inspection (S1, S5)

1. Clean the area first. Much of what is found is found while cleaning.
2. Work the prone areas predicted by GEN-G Concepts §1 — low points, drains, faying surfaces, dissimilar-metal joints, battery and lavatory areas, anywhere a coating is broken.
3. Use raking light and aided vision (GEN-K Procedures Procedure 2).
4. Look for the signs, not just the metal: blistered or lifted paint, white or gray powder, staining, pillowing of skins between fasteners, smoking rivets.
5. Establish the extent, not just the presence.
6. Classify the type — surface, pitting, intergranular, exfoliation, filiform, galvanic, stress — because the type determines the treatment and the seriousness.
7. Record location, type, extent and depth.

Check — blistered paint is a finding until proven otherwise. Corrosion under a coating lifts it, so any lifted or blistered finish must be opened up and looked at rather than noted and painted over.

Failure — recording that corrosion is present and not how far it goes. The treatment then stops at the visible edge (GEN-G Concepts §7).

Procedure 2 — Remove and treat corrosion (K7, supporting S1)

1. Confirm the allowable removal depth from approved data before starting. There is a limit, and past it the part is repairable or scrap rather than treatable.
2. Protect the surrounding area and adjacent components.
3. Remove by the approved method for that material — non-metallic abrasive on aluminum, never steel wool or a steel brush, which embeds particles and creates a galvanic couple.
4. Blend the removal out to a smooth contour. Sharp edges concentrate stress.
5. Verify all corrosion is gone — inspect, and re-inspect after cleaning.
6. Measure what you removed and compare against the limit.
7. Treat: conversion coating, then primer, then the approved finish system.

Check — measure the depth removed and state it against the limit. Removal without measurement is how a serviceable part becomes an unserviceable one with no record of why.

Failure — a steel brush on aluminum. It removes the corrosion and seeds the next one.

Procedure 3 — Apply corrosion-preventive compound (S3)

1. Surface clean and dry. CPC over contamination traps it.
2. Select the compound by approved data — they differ in thickness, penetration, water displacement and whether they are meant to be removed later.
3. Apply by the specified method to the specified coverage.
4. Keep it off what it must not touch: brakes, discs, tires, windows, control surfaces where balance matters, and anywhere a placard or marking must stay legible.
5. Allow the specified cure or set time.

Check — check coverage in the places you cannot see — inside lap joints, behind stiffeners, in the drain path. That is where it is needed and where it is missed.

Failure — blocking a drain hole with compound. You have added an electrolyte trap while protecting against electrolyte.

Procedure 4 — Prepare a metal surface for painting (S11)

1. Remove all old finish where the approved process requires it, by the approved method.
2. Degrease — and degrease before abrading, not after, or you drive contamination in.
3. Abrade to the specified profile with the specified abrasive.
4. Clean again and confirm the surface is free of residue.
5. Apply etch and conversion coating per Procedure 5.
6. Prime within the specified window. A prepared surface starts oxidizing immediately, and the window exists for that reason.

Check — the water-break test. Clean water sheets evenly across a properly cleaned surface; it beads where contamination remains. It takes seconds and it is the only direct evidence you will get before the finish tells you months later (GEN-G Concepts §5).

Failure — priming a surface that failed the water-break test. The finish looks perfect and lifts within a year.

Procedure 5 — Apply etch solution and conversion coating (S13)

1. Check ambient conditions against the product's window — temperature and humidity both.
2. Apply etch for the specified dwell, and no longer. Over-etching removes more metal than intended.
3. Rinse thoroughly. Residual etch under a coating continues to work.
4. Apply conversion coating for its specified dwell.
5. Rinse and dry per the instructions.
6. Inspect the color and uniformity — conversion coatings develop a characteristic appearance, and patchiness means uneven coverage or a contaminated surface.

Check — the coating's color is the process indicator. A pale or uneven result means the dwell, the concentration or the cleaning was wrong, and the answer is to reprocess rather than to proceed.

Failure — leaving etch on beyond the dwell because something interrupted you. Set a timer.

Procedure 6 — Prepare a composite surface for painting (S8)

Different from metal, and the differences matter.

1. Confirm the approved process for that laminate. Composites vary and so do their coatings.
2. Abrade lightly and by hand, only enough to key the surface. Do not expose or damage fibers — you are preparing a surface, not removing material, and cut fibers are structural damage.
3. Never use a metal-appropriate chemical stripper unless it is approved for the laminate; most attack the matrix.
4. Clean with the approved solvent and allow full flash-off.
5. Inspect for exposed fiber, scratches into the laminate, and moisture.

Check — look for shiny fibers. If you can see fiber, you have gone through the surface resin and you have a repair, not a preparation.

Failure — abrading a composite as though it were metal. It removes the resin-rich surface layer that is doing the protecting.

Procedure 7 — Lay out and mask a surface (S10, with S7)

1. Establish the required registration marking size, location and legibility from 14 CFR part 45 and confirm what the aircraft requires. The size rules are proportions, and they are in 45.29:

   • Height — at least 12 in for fixed-wing aircraft and for rotorcraft; at least 3 in for airships, balloons and powered parachutes. Fixed-wing carries exceptions, so establish the category and check them before applying any height.
   • Width — two-thirds of the height, 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. The 1 exception is mandatory; the M/W one is a permission.
   • Thickness — solid lines one-sixth as thick as the character is high.
   • Spacing — not less than one-fourth of the character width. Width, not height, and width differs by character.
   • Both sides must match on a fixed-wing aircraft — same height, width, thickness and spacing.
   • Where neither surface is large enough for full-size marks, marks as large as practicable are required. The rule shrinks the marking; it never excuses it.

2. Measure and mark from a datum on the aircraft, not from the previous marking, which may itself be wrong.

3. Apply masking, burnishing the edges down so the coating cannot creep under.

4. Mask what must not be coated: static ports, pitot heads, drain holes, vents, antennas, placards, hinge lines, seals.

5. Remove masking at the time the coating system specifies — usually while the finish is still soft, or the edge tears.

Check — walk the aircraft and list what you masked. Static ports and drains are the two that get missed, and both cause problems that are not discovered until flight.

Failure — masking measured from the old marking. Errors are inherited and compound.

The checks, collected

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