When an aircraft finish reaches the end of its service life, shows signs of adhesion failure, or must be removed before a major structural repair, the Aviation Maintenance Technician (AMT) must choose the right removal method to strip the old coating without damaging the skin, frame, or underlying protective treatment beneath it. Two of the most common mechanical approaches are sanding and abrasive blasting. Unlike chemical stripping, which relies on solvents and paint removers to dissolve the binder in a coating, mechanical methods use physical abrasion to cut, grind, or impact the coating away from the surface. Each method carries distinct advantages, limitations, and procedural requirements that every AMT must understand both for practical shop work and for the FAA Airframe Knowledge Test.
This article examines both techniques in depth — how they work, when to use each, the equipment and media involved, the critical safety and substrate-protection concerns, and the key facts the FAA expects you to know.
Why Paint Removal Matters in Aircraft Maintenance
An aircraft's finish system is not simply cosmetic. The primer layer, topcoat, and any underlying conversion coating work together as a corrosion-control system. When a coating breaks down — showing blistering, peeling, cracking, or loss of adhesion — corrosion can begin rapidly underneath it, especially on aluminum alloy structure. The AMT must remove degraded coatings completely before applying a new finish system; painting over failing paint traps moisture and accelerates the very corrosion the new finish is meant to prevent.
Equally important, the removal process itself must not damage the base metal. Aluminum alloys used in aircraft skins are relatively soft, and aggressive abrasion can remove or thin the metal, create stress risers, or destroy the very thin anodized or chemically treated (Alodine/chromate conversion) surface layer that provides the first line of corrosion resistance. This is why mechanical paint removal on aircraft demands careful technique, correct media selection, and ongoing inspection of the work surface.
Sanding: Technique, Equipment, and Applications
Sanding uses abrasive particles bonded to a flexible backing — sandpaper or abrasive discs — to mechanically abrade a coating off the surface. It is among the most controllable of all removal methods, allowing the technician to work in a small, precise area and to stop at exactly the right layer.
Types of Abrasives and Grit Selection
Sandpaper is rated by grit number: the higher the number, the finer the abrasive and the smoother the finish it leaves. For paint removal on aircraft:
- Coarse grits (80–120) cut quickly and are used for initial removal of thick or multiple coats, but they leave deep scratches that must be blended out.
- Medium grits (150–220) are a compromise often used for general stripping of single topcoats or for feathering the edges of a repair area.
- Fine grits (320 and above) are used for final scuff-sanding before primer application, not for bulk paint removal.
For aluminum aircraft skin, it is critical to avoid very coarse abrasives (below 80 grit) in power tools, as these can rapidly remove base metal. The AMT's Handbook (FAA-H-8083-31) emphasizes that sanding should be performed carefully to avoid cutting through the primer and into the conversion coating.
Hand vs. Power Sanding
Hand sanding using a sanding block gives the technician the best feel for surface pressure and is preferred on thin aluminum skins, around rivets, and in areas where compound curves make it easy to cut through the metal with a power tool. Power sanders — orbital, dual-action (DA), or straight-line — speed up removal on large flat panels but require constant attention to avoid dwelling in one spot. A dual-action (DA) or random-orbital sander is preferred over a straight-line sander because it minimizes the risk of creating sanding grooves aligned with the grain of the metal, which can act as stress concentration points.
When Sanding is the Right Choice
Sanding is well suited for: spot repairs where only a localized area needs stripping; thin-gauge aluminum or composite surfaces where blast pressures would cause damage; areas around control hinges, seams, or fasteners where precision is needed; and touch-up work on field repairs. It is typically too slow and labor-intensive for stripping an entire airframe.
Abrasive Blasting: Technique, Equipment, and Applications
Abrasive blasting propels a stream of abrasive media against the surface using compressed air (or, in wet-blast systems, a water-air-media mixture). The high-velocity impact fractures and dislodges the coating. Blasting can strip large areas quickly and can reach into recesses, rivet heads, and lap joints that are difficult to sand by hand.
Blasting Equipment
The basic blast system consists of a pressure pot (a pressure vessel that holds the media and feeds it into the air stream) or a suction (siphon) blast gun, connected to a compressor and a blast nozzle. Pressure-pot systems are faster and more aggressive than siphon systems and are used for production work. The nozzle size, air pressure, standoff distance (distance from nozzle tip to the surface), and angle of impingement are all adjustable parameters that dramatically affect how aggressively the system cuts.
Blasting Media
Media selection is one of the most critical decisions in abrasive blasting on aircraft. Using the wrong media can peen, warp, or erode the base metal, or leave contaminants embedded in the surface that cause adhesion failure of the new finish.
- Glass beads: A very common choice for aluminum aircraft. Glass beads produce a smooth, clean, peened surface with minimal metal removal. They are spherical and therefore compressive rather than cutting in their action, making them relatively safe for aluminum skin. They must be used at low pressures (typically 20–40 psi for thin aluminum).
- Aluminum oxide: Sharp, angular media that cuts aggressively. More suitable for steel parts, heavy structure, or hard metals. Can remove too much material from thin aluminum skins and must be used with care and at low pressures if used on aluminum at all.
- Plastic media (plastic bead blasting / PMB): Soft, angular plastic particles (such as urea, acrylic, or melamine) are specifically designed for aerospace paint removal. They are hard enough to cut paint but soft enough that they remove little or no metal. Plastic media blasting (PMB) has become a widely accepted aerospace industry standard for stripping aircraft because it does not damage aluminum, composites, or the conversion coating beneath the primer when used correctly.
- Walnut shell and corn cob: Organic, soft media sometimes used on composite structures where even plastic media might cause surface damage. Very gentle.
- Steel shot and steel grit: Used on steel parts and engine components, never on aluminum airframe skins, as they will contaminate the aluminum with embedded iron particles that cause galvanic corrosion.
Blast Pressure and Standoff Distance
These two variables control the energy delivered to the surface. For aluminum structure, pressures are kept low — often between 20 and 60 psi depending on media and skin thickness — and the nozzle is held at an angle (typically 30–60 degrees from the surface rather than perpendicular) and kept moving. Dwelling in one spot, using too high a pressure, or blasting perpendicularly can cause metal erosion, warping of thin skin panels, and media embedment. Skin thickness must be monitored with an ultrasonic thickness gauge before and after blasting on thin structural panels.
Key Numbers and Rules
- Always consult the aircraft manufacturer's Structural Repair Manual (SRM) and the coating manufacturer's data sheet before removing paint; some processes are required or prohibited for specific aircraft.
- Plastic media blasting pressures for aluminum skin are typically kept below 40–60 psi; always verify with the applicable process specification.
- After mechanical removal, the surface must be inspected for bare metal, corrosion, or damage to the conversion coating before priming.
- Bare aluminum exposed by sanding or blasting must be re-treated with a chromate conversion coating (Alodine) or equivalent before applying primer, because the mechanical process removes the original surface treatment.
- Composite structure (fiberglass, carbon fiber) requires especially gentle media (plastic or organic) and very low pressures to avoid delamination or fiber damage.
- Blasting media that has been contaminated with oil or moisture must be discarded; contaminated media can embed impurities in the surface and ruin adhesion.
Common Test Traps
- Assuming sanding is always safe for aluminum: Coarse grits in power tools can rapidly remove base metal and destroy the conversion coating. Grit selection and technique matter as much as method choice.
- Confusing media types: The FAA expects you to know that steel shot is never used on aluminum skins due to galvanic corrosion risk, and that plastic media is the aerospace-preferred blasting medium for aluminum.
- Forgetting surface re-treatment: Mechanical paint removal always exposes bare metal. The test may ask what must be done before priming — the answer is to re-apply a conversion coating (Alodine treatment) to restore corrosion protection.
- Overlooking composite vulnerability: Students often apply aluminum rules to composites. Blasting pressures and media safe for aluminum can delaminate or fracture composite skins. Always specify softer media and lower pressures for composites.
- Neglecting documentation and manuals: The FAA stresses following the manufacturer's SRM and applicable specifications. Choosing a removal method based on habit rather than approved data is a common real-world — and test — trap.