Checklist for Accepting Coated Metal Components

Start With Visual Inspection

Before you touch a caliper or pull a sample, look at the parts. Light the components under diffuse illumination. Check the entire surface, not just the top of the parts. Turn each piece so the light rakes across the coating. This reveals bumps, pits, and uneven thickness that flat lighting hides.

Look for these common visual defects:

1. Spatter or splatter: Small drops of molten material or solvent residue. Common on powder coating lines and spray operations.
2. Runs or sags: Coating that flowed downward during cure or application. Typical in high solids liquid coatings.
3. Bridging: Thin film between two edges or features. Usually caused by incorrect application speed or part spacing.
4. Pinholes: Tiny holes in the film. Often linked to surface prep issues or solvent traps.
5. Blisters or bubbles: Trapped gases or moisture. A sign of poor substrate cleaning or humidity control.
6. Color variation: Mismatched shade between parts in the same lot. Can stem from batch changes or temperature drift.

Red flags include defects on visible surfaces, defects on mating surfaces, or any defect that affects the functional fit of the part. If a scratch or chip exposes bare metal, that is a reject unless the drawing allows it.

Verify Coating Thickness and Uniformity

Visual checks do not tell you if the coating meets spec. Measure thickness at multiple points on each part. Use a calibrated gauge appropriate for the coating type. For metallic coatings, use eddy current or ultrasonic methods. For organic coatings, use a standard coating thickness meter.

Take readings at least three times per part, at different locations. Record the minimum, maximum, and average. Compare results to the specification. If the drawing calls for a minimum thickness, ensure the thinnest point meets it. If the spec gives a range, ensure no point falls outside it.

Consider the geometry of the part. Corners and edges often have thicker coatings. Recesses and hollows may be thinner. A part that measures within spec on a flat face may fail on an edge.

Red flags include:

• Thickness below spec at any point.
• Large variation between parts in the same lot.
• No thickness data provided with the delivery.
• Using a gauge that is not calibrated or out of date.

If you cannot measure thickness, ask for the supplier’s test report. If the report is missing or vague, treat the lot as suspect.

Run Adhesion Testing

Adhesion testing confirms that the coating stays on the substrate. Do not skip this step. A coating can look perfect and still peel off during use. The method you choose depends on the coating type and the part size.

For small parts, the cross-cut test is common. Score the surface with a blade in a grid pattern. Apply tape or pressure and remove. Evaluate how much coating remains. A standard scale exists for this test, but your specification may define pass criteria.

For larger parts or thicker coatings, the pull-off test is more definitive. An adhesive anchor is glued to the coating. A machine pulls the anchor off, measuring the force required. This gives a numerical value in megapascals or pounds per square inch.

For parts that cannot be tested destructively, ask the supplier for pull-off data from a sample batch. If the parts are critical, consider a more rigorous test.

Red flags include:

• Peeling or lifting edges on any part.
• Cross-cut scores showing more than a few percent of the coating lifted.
• No adhesion test data for the specific lot.
• Using a test method that does not match the coating type.

If adhesion fails, do not approve the lot. Return the parts or require rework. Poor adhesion is often linked to surface prep, which is hard to fix after the fact.

Check Surface Preparation Records

Coating quality depends on what happens before the coating goes on. Ask for surface preparation records. Look for evidence of cleaning, degreasing, and conversion treatment. For chromate or zinc plating, the pretreatment chemistry matters. For powder coating, the cleaning method matters.

Review the process log. Note the cleaning agent used, the concentration, and the rinse water quality. If the supplier uses a specific pretreatment, ask for the lot traceability of the chemicals.

Red flags include:

• No record of cleaning or degreasing.
• Inconsistent cleaning times or temperatures.
• Use of unknown or unapproved chemicals.
• High humidity or dust during processing.

If the records are incomplete, ask for clarification. Do not assume the parts are good because they look good.

Review Documentation and Traceability

A good finish acceptance criteria set includes paper checks. Before approving the delivery, verify that all documents are present and accurate.

Check the packing list. Ensure the part numbers, quantities, and descriptions match the purchase order. Check the material certificate. Confirm the substrate material matches the spec. Check the coating lot number. Ensure it is traceable to the specific supplier and date.

Review the test reports. Ensure the thickness and adhesion data match the parts you are inspecting. If the data is for a sample batch, confirm that the sample was taken from the same lot as the parts being delivered.

Red flags include:

• Missing or illegible documents.
• Lot numbers that do not match the parts.
• Test data that does not match the part numbers.
• No certification of compliance with the specification.

If documentation is missing, hold the parts. Do not approve them until you have complete records.

Define Acceptance Criteria

Before you start inspecting, know what you are looking for. Finish acceptance criteria should be written down. They should be specific and measurable. A good criteria set includes:

• Visual defect limits (size, location, quantity).
• Coating thickness range and number of measurement points.
• Adhesion test method and pass value.
• Color match method, if applicable.
• Surface roughness requirements, if applicable.
• Corrosion test requirements, if applicable.

If the drawing does not specify all of these, ask the supplier for their standard acceptance criteria. If the supplier does not have a written standard, define one.

A clear criteria set prevents disputes. If both parties know what is acceptable, you avoid arguments about minor scratches or slight color variation.

Final Approval and Records

Once you have completed all checks, make a decision. If the parts meet all finish acceptance criteria, approve the delivery. If they do not, reject them. Document the reason for rejection. Include photos, test data, and part numbers.

Keep the inspection records with the purchase order. Store them for the life of the part. If the part fails later, you will need to trace back to the coating lot and the inspection records.

A simple approval form can help. Include fields for part number, quantity, inspector name, date, and pass/fail status. Add a space for notes and any nonconformance details.

If you approve the parts, send the approval to the supplier. If you reject them, send the rejection notice with supporting evidence. This creates a clear paper trail.

Common Mistakes to Avoid

• Approving parts based on visual inspection alone.
• Skipping adhesion testing on critical parts.
• Not measuring thickness at edge and corner locations.
• Accepting vague or missing documentation.
• Failing to define finish acceptance criteria before inspection.
• Not storing inspection records for future reference.

These mistakes lead to costly failures. A small defect today becomes a product failure tomorrow.

When to Involve the Supplier

If you find a defect, contact the supplier immediately. Provide photos, part numbers, and test data. Ask for a root cause analysis. Do not just say “this is bad.” Explain what the defect is and where it is located.

If the supplier cannot explain the cause, escalate the issue. Request a corrective action plan. Do not accept future lots from the same lot number until the issue is resolved.

If the issue is recurring, review your supplier selection process. You may need to change suppliers or tighten your specifications.

Conclusion

Finish acceptance criteria protect your business. They ensure that the parts you approve meet the quality you need. Use this checklist to standardize your inspection process. It will save time, reduce disputes, and improve product reliability.

Start with the simplest checks. Move to technical tests. Review documentation. Make a clear decision. Keep your records. Do this every time, and you will reduce the risk of costly failures.