Stainless surface finishing: brushed, mirror and blasted — process and acceptance
Brushed (No.4) is judged on grain uniformity and direction, mirror (No.8) on Ra and absence of scratches, blasting on uniformity and roughness. Finish affects both appearance and corrosion.
Three surface finishes are common for stainless appearance parts:
1) Brushed (No.4 / hairline): unidirectional grinding with abrasive belts or flap wheels. Acceptance checks grain straightness, uniformity, colour consistency and direction; keep the same direction within a batch.
2) Mirror (No.8): multi-stage polishing to roughly Ra ≤0.1 μm depending on requirements. Acceptance checks reflection clarity, absence of scratches and polishing compound residue; mirror surfaces show fingerprints and scratches, so protective film is needed.
3) Blasted (sand/bead blasting): uniform matte surface. Acceptance checks roughness (Ra), uniformity and colour difference; blasting changes the passive state, so post-blasting passivation is usually required.
Guidance:
• Specify the finish code and an approved sample to avoid “close enough” disputes.
• Agree the brushing direction in the order.
• Protect mirror surfaces with film during transport.
• After brushing/blasting, passivate and run a blue-dot test where corrosion resistance matters.
Surface finish affects appearance, corrosion resistance and cleanability — write the acceptance criteria into the technical agreement.
