Finishing & Quality · SOP

Pre Finish Sanding Scratch Depth Standard Reference Samples

Standardised procedure for creating, measuring, and maintaining reference samples that define acceptable scratch depth for each grit of sandpaper used in pre-finish operations. Ensures repeatable surface quality and reduces rework.

Estimated Time

45 min

Difficulty

Master

Regulation

ISO 9001 / Custom QA

Version

1.0

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Purpose

To establish a set of physical reference samples that visually and measurably represent the correct scratch depth for each grit of sandpaper used before final coating. These samples enable operators to verify that their sanding technique, pressure, and tool condition produce a consistent scratch pattern within the defined quality limits. Without standardised references, scratch depth variation leads to coating adhesion failures, orange peel, and unacceptable finish appearance.

Scope

This SOP applies to all pre-finish sanding operations in the wood products and coating facility, including MDF, plywood, and solid wood substrates. It covers the creation of initial reference samples for grit sizes P120, P150, P180, P220, P280, P320, and P400, as well as the periodic verification and recalibration protocol. Excludes sanding after primer or sealer coats (post-finish sanding) – those are covered under SOP-FIN-102.

Safety Precautions

PPEWear safety glasses and a NIOSH-approved dust mask (N95 or better) when sanding. Fine dust is a respiratory hazard.
HANDKeep hands clear of moving sanding tools. Use a backing block or pad—never hold sandpaper directly in your palm during machine sanding.
ELECTIf using an orbital sander, inspect the cord and plug for damage. Unplug before changing paper.
CHEMTack cloths may contain solvents. Use in a ventilated area and dispose of used cloths in a sealed metal container.

Required Tools & Materials

Substrate panels: MDF, plywood, or solid wood (300 mm × 200 mm × 18 mm min), one per grit
Assorted sandpaper: P120, P150, P180, P220, P280, P320, P400 (aluminium oxide or stearate coated, closed-coat)
Sandpaper backing block (rubber or cork, 90 mm × 180 mm) – manual method
Orbital sander (optional, for production speed) with variable speed control
Scratch depth gauge / surface profilometer (e.g., Mitutoyo SJ-210, accuracy ±0.1 µm)
Digital calliper or micrometer (for checking panel thickness uniformity)
Tack cloth (lint-free) or compressed air (max 15 psi) for dust removal
Protective sleeves or sealed cases for sample storage
Permanent marker and labels (grit, date, operator, measured depth)
Calibration certificate for depth gauge (traceable to NIST or equivalent)

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Procedure: Step-by-Step

1 Select clean, defect-free substrate panels (MDF, plywood, or solid wood) of uniform size and grain direction. Label each panel with grit designation (e.g., "P120 REF"). Ensure panel thickness varies less than 0.2 mm across the surface.

2 Mount a fresh sheet of the designated grit sandpaper on the backing block or sander. If using an orbital sander, set speed to 8000–10000 OPM (or equivalent) and affix paper with hook-and-loop backing.

3 Sand the panel using consistent pressure (approx. 1 kg–2 kg downward force) and direction (with the grain) for a fixed number of strokes: 10 back-and-forth passes (manual) or 15 seconds of orbital motion. Apply uniform contact over the entire surface to avoid localised depth variation.

4 Clean the sanded surface thoroughly with a tack cloth or compressed air (blow from one side to prevent re-deposition). Inspect under strong oblique lighting for any visible scratches or embedded grit. If contamination is present, repeat from Step 3 using a clean paper.

5 Measure the scratch depth using the surface profilometer. Calibrate the gauge per manufacturer instructions with the provided standard. Take three measurements at random locations within the sanded area (avoid edges). Record the average scratch depth (Ra or Rz, as determined by QA). Acceptable range per grit: refer to Table 1 (internal document QA-TBL-01).

6 If the measured depth falls outside the acceptable range, adjust sanding pressure (lighter for deeper scratches, heavier for shallower) or number of passes. Repeat from Step 3 until the depth is within tolerance. Record all iterations on the sample creation log.

7 Once acceptable, seal the reference sample in a protective sleeve or case. Label with grit size, date of creation, operator name, measured scratch depth (Ra/Rz), and calibration due date. Store in a climate-controlled cabinet (temperature < 30°C, relative humidity < 60%) away from direct sunlight.

8 Perform weekly verification: compare each reference sample against a master sample (the original creation for that grit) using the profilometer. Recalibrate if deviation exceeds 5% of original depth. Recreate the sample if deviation exceeds 10% or if the sample shows physical wear, staining, or damage.

9 Document all creation, verification, and recalibration events in the electronic quality log (ServiceGrid CMMS or equivalent). Tag the procedure line item with “Scratch Depth Ref Samples” for trending analysis.

Pre Finish Sanding Scratch Depth Standard Reference Samples

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