Stainless Steel Surface Roughness (Ra) vs Sanding Belt Grit Guide （Polishing Stainless Steel）

When polishing 304 stainless steel, the achievable surface roughness (Ra) depends not only on the sanding belt grit size, but also on factors such as polishing equipment, applied pressure, speed, lubrication (dry vs wet grinding), and the condition of the initial surface.

Below is an industry experience-based grit-to-Ra reference chart for stainless steel:

Key Notes and Influencing Factors

• Surface Roughness Parameters:

  • Ra (arithmetical mean roughness) is the most common measure, showing the average deviation from the mean surface line.

  • Rz (maximum height roughness) measures the vertical distance between peaks and valleys and is usually several times larger than Ra.

• Dry vs Wet Grinding: Wet polishing provides finer results, reduces clogging, and helps with cooling.

• Equipment Matters: Professional belt grinders with constant belt speed achieve more consistent finishes than hand-held tools.

• Step-by-Step Progression: Do not skip grit sizes. Progress gradually (e.g., 220 → 320 → 400 → 600 → 800). Jumping too fast leaves deep scratches behind.

• Pressure and Speed: Excessive force or speed can create deeper scratches. Uniform pressure and steady speed are critical.

• Abrasive Material: Silicon carbide (SiC) and aluminum oxide (Al₂O₃) belts have different cutting behaviors. Choose based on application.

Brushed Stainless Steel Finishes

Common brushed stainless steel effects such as hairline or satin finishes are typically achieved using grits from 180 to 600, producing Ra values in the 0.4–1.6 μm range. These finishes are widely used in decorative applications, elevators, appliances, and architectural stainless steel.

Practical Recommendations

• Industrial Applications: Always follow process specifications and verify results with a roughness tester.

• Workshops & DIY: Test on scrap material first. Start with lower grit and move step by step until the desired finish is achieved.

• Mirror Finish: To achieve Ra ≤0.1 μm, sanding belts up to 2000 grit should be followed by buffing with polishing compounds and cloth wheels.

In conclusion, while grit size strongly influences Ra, achieving a consistent and high-quality stainless steel surface requires proper equipment, controlled parameters, and stepwise grit progression.