What finishing processes are available for aerospace-grade castings?

Available finishing processes for aerospace aluminum castings include cleaning and conversion pretreatment, anodizing where the alloy and function permit, primers and liquid paints, selected powder coatings for suitable equipment, plating or specialized metallic layers, sealers, controlled bare or machined surfaces, and mechanical preparation such as blasting, tumbling, or polishing. The drawing must replace "aerospace-grade finish" with the exact authorized specification and revision, type/class, full layer stack, pretreatment, thickness, color, masks, cure, tests, approved source, records, and repair limits.

Start with the substrate and function

Identify alloy specification, casting route, heat treatment, silicon content, porosity/outgassing risk, surface condition, and subsequent joining or assembly. A356, AlSi10Mg-type, A360, and other cast alloys do not respond identically to cleaning, etching, anodizing, or paint preparation. Wrought-aluminum process assumptions should not be transferred without qualification.

Map corrosion barrier, wear surface, electrical ground, EMI joint, thermal contact, bond area, seal land, bearing fit, thread, fastener seat, identification, cosmetic face, and fluid-wetted zone. One coating cannot serve every function. Define expected humidity, salt, fluids, cleaning agents, UV, atomic oxygen or vacuum where applicable, temperature, abrasion, fretting, and service damage.

Use conversion, anodizing, and paint as complete systems

Conversion treatments can prepare aluminum and provide corrosion or electrical functions when the authorized chemistry and process are followed. Anodizing can support corrosion, wear, appearance, or insulation on suitable cast surfaces, but high-silicon content, porosity, geometry, contact points, and dimensional buildup affect the result.

Primer and paint systems can provide barriers, color, identification, cleanability, and fluid resistance. Select the full approved system, not merely epoxy or polyurethane family. Mixing pretreatment, primer, topcoat, thinner, cure, or source outside authorization can invalidate evidence.

Powder coating may suit selected noncritical housings, cabin or ground equipment, and other applications where its film, cure, repair, flammability, outgassing, weight, edge coverage, and electrical/thermal effects are accepted. It is not automatically an aerospace replacement for an approved liquid system.

Control metallic layers, sealers, and bare zones

Plating or other metallic/conductive finishes may be specified for electrical contact, wear, corrosion, or mating functions. Cast substrate quality, pretreatment, adhesion, hydrogen or embrittlement concerns for attached materials, galvanic couples, porosity, and thickness distribution need review. Use only the program-approved process and source.

Sealers or impregnation can address selected surface or leak functions but must be separately authorized. Define chemistry, zone, vacuum/process control, cure, compatibility, inspection, identification, and repeat treatment. They cannot be used to conceal an unapproved structural or pressure-integrity condition.

Bare, conversion-coated, masked, plugged, machined-after-finish, and touch-up zones should appear on controlled data. Protect electrical grounds, bond surfaces, bearing seats, threads, seal lands, thermal interfaces, and datum targets. Film under a fastener can change preload; a mask edge can create a corrosion site.

Treat mechanical preparation as a controlled step

Abrasive blasting, tumbling, deburring, and polishing remove scale or burrs and create texture; they are not corrosion protection by themselves. They can expose pores, embed media, close or open surface indications, round edges, damage thin walls, change fatigue-sensitive surfaces, and contaminate passages.

Specify media type, contamination controls, size, pressure or intensity, coverage, time, masks, surface profile, cleaning, and acceptance. Separate equipment may be required to avoid cross-contamination. Inspect at the correct stage when penetrant or bonding follows preparation.

Sequence machining, cleaning, finishing, and NDT

Machining can expose discontinuities and creates burrs, chips, coolant, and fresh surfaces. Heat treatment can distort or oxidize parts. NDT may be required before and/or after selected operations. Finishing can hide surfaces from later inspection or alter dimensions. Create a process flow that explains why each stage occurs where it does.

Cleaning must remove release agent, oil, coolant, oxide, polishing compound, salts, and abrasive without attacking the substrate or leaving residue. Control rinse quality, drying, handling, queue time, and packaging. Bond and paint failures often begin with invisible contamination.

Verify the finish by function

Qualify sources and control repair

Chemical processing, anodizing, painting, plating, bonding, and NDT may require customer-approved or accredited sources, qualified procedures, equipment, personnel, panels, and records. Verify the actual facility and process scope. General finishing capability is not proof of aerospace approval.

Define touch-up and stripping limits. Repair can create thickness steps, trapped chemistry, dimensional change, substrate attack, fatigue damage, color difference, or lost traceability. State who may approve repair, the procedure, repeat limit, inspection, marking, and when the part must be rejected.

What buyers should provide

Send alloy/route/condition, part classification, environment, function-by-zone map, exact finish documents and revisions, approved sources, process sequence, masks, dimensions after finish, color/appearance samples where applicable, NDT, qualification, production tests, repair, packaging, traceability, records, and change controls.

The available finish is only relevant when it is authorized for the substrate, installation, source, and complete process sequence. That controlled definition, not the phrase "aerospace-grade," protects durability and configuration.