Begin With the Specification

Before testing anything, determine what the finished coating is required to meet.

The requirements may come from:

• The powder manufacturer’s technical data sheet
• A customer purchase order
• An engineering drawing
• An architectural specification
• An approved color standard
• An industry test standard
• An internal quality plan
• A qualified coating procedure

The quality plan should define:

• The test method
• The test frequency
• The test location
• The required equipment
• The environmental or conditioning requirements
• The acceptance criteria
• Who may perform and approve the test
• What happens when a result fails

Use Representative Test Surfaces

Testing may be performed on production parts, witness panels, laboratory panels, or a combination of these.

Production Parts

Production parts provide the most direct evidence of what the actual process produced. However, some tests damage the coating and may make the part unacceptable for shipment.

Witness Panels

A witness panel travels through the process with the production parts. It can be used for destructive testing without damaging the finished product.

The panel should represent the production process as closely as practical:

• Same or approved substrate
• Same surface preparation
• Same pretreatment
• Same powder
• Same application conditions
• Same oven cycle

A perfectly prepared laboratory panel does not always represent a complex production part with welds, heavy sections, rack shadows, and varying film thickness.

Allow the Coating to Cool and Condition

A part leaving the oven may be too hot to test safely or accurately. Coating properties can also change as the film cools and completes its post-oven conditioning.

Before testing:

• Allow the part to cool to the temperature required by the procedure.
• Condition the sample for the specified time when required.
• Protect the test surface from fingerprints, oil, dirt, and damage.
• Confirm that the testing environment meets the applicable procedure.

Adhesion Testing

Adhesion testing evaluates how well the cured coating remains attached to the substrate under a defined cutting and tape-removal procedure.

Common tape-adhesion procedures include:

• An X-cut made through the coating
• A cross-cut or lattice pattern made through the coating

The required method depends on the coating, film thickness, substrate, specification, and applicable standard.

General Tape-Test Sequence

1. Select a suitable test area.
2. Measure and record the film thickness.
3. Cut through the coating to the substrate using the specified cutting tool and pattern.
4. Remove loose material as required by the procedure.
5. Apply the specified pressure-sensitive tape.
6. Press the tape into contact using the approved method.
7. Remove the tape at the required angle, speed, and time.
8. Inspect the cut area and tape.
9. Rate the result using the specified classification system.
10. Photograph and document the test when required.

Use the tape, cutting tool, spacing, and technique required by the governing test method. Substituting ordinary shop tape can make the results impossible to compare.

What an Adhesion Failure May Mean

Poor adhesion can result from:

• Oil, dirt, or other surface contamination
• Inadequate cleaning
• Incorrect pretreatment
• Poor rinse-water quality
• Pretreatment residue
• Flash rust
• Incomplete cure
• Excessive film thickness
• Incompatible powder or substrate
• Poor intercoat adhesion in a multiple-coat system
• Incorrect test technique

The location of the separation matters. Determine whether the coating separated from the substrate, within the pretreatment layer, between coating layers, or within the coating itself.

Cure Verification

Cure verification helps determine whether the powder coating received enough thermal exposure to develop its intended properties.

Possible methods include:

• Review of an oven profile
• Solvent-resistance testing
• Hardness testing
• Impact or flexibility testing
• Adhesion testing
• Laboratory analysis
• Powder-manufacturer-specific procedures

An oven profile proves the recorded time-and-temperature history at selected locations. A coating test evaluates a property of the resulting film. A dependable quality program may use both.

Solvent-Resistance Testing

A solvent-rub test may provide a quick indication of cure for certain powder coatings. The procedure applies a specified solvent to the cured film using a controlled number of rubbing cycles.

The written procedure should identify:

• The solvent
• The cloth, applicator, or test device
• The wetting method
• The number of double rubs
• The applied pressure
• The test location
• The evaluation method
• The acceptance criteria

The coating may be evaluated for:

• Softening
• Color transfer
• Gloss change
• Film removal
• Surface damage

Different powder chemistries have different solvent resistance. A procedure developed for one powder should not automatically be applied to another.

Hardness Testing

Pencil-hardness testing evaluates the resistance of the coating surface to scratching or gouging by prepared pencils of graded hardness.

Results can be affected by:

• Pencil preparation
• Test angle and applied force
• Operator technique
• Film thickness
• Substrate flexibility
• Surface texture
• Coating temperature
• Powder formulation and cure

The test method and acceptance criterion should specify whether the reported value is gouge hardness, scratch hardness, or another defined result.

Do not use pencil hardness alone to diagnose cure. A coating formulation may be designed to remain flexible while still being properly cured.

Gloss Measurement

A gloss meter directs light toward the coating at a controlled angle and measures the reflected light. The result is reported in gloss units.

The appropriate measurement geometry depends on the coating and specification. Common geometries use different incident angles for high-, medium-, and low-gloss surfaces.

For repeatable results:

• Use the specified measurement angle.
• Verify the instrument with its reference standard.
• Keep the standard and measurement optics clean.
• Measure a flat area large enough for the instrument.
• Use a consistent orientation on directional or textured finishes.
• Allow the part to cool and condition.
• Take the required number of readings.
• Record individual values and averages as specified.

What Can Change Gloss?

• Powder formulation
• Color and pigmentation
• Film thickness
• Surface texture
• Cure temperature and time
• Uneven oven heating
• Powder contamination
• Mixing incompatible powders
• Substrate profile
• Measurement angle and technique

Gloss variation across one part may point toward uneven film thickness, oven exposure, application, or substrate condition. Variation between batches may indicate a powder, cure, reclaim, or process change.

Color Measurement

A colorimeter or spectrophotometer can compare a coated surface with an approved standard. Instrumental color is commonly expressed using coordinates such as L*, a*, and b*.

• L*: Lightness from dark to light
• a*: Position along a red-to-green axis
• b*: Position along a yellow-to-blue axis

A calculated color difference may be reported as Delta E. The particular Delta E formula and acceptance limit must be specified. Different formulas can produce different numerical results.

Color readings are also affected by:

• Instrument geometry
• Illuminant setting
• Observer setting
• Measurement aperture
• Gloss and texture
• Metallic or special-effect orientation
• Film thickness
• Substrate color
• Surface curvature

Metallic and Special-Effect Colors

Metallic, mica, bonded, hammertone, vein, sparkle, and other effect finishes may change appearance with viewing angle, application settings, reclaim content, and part orientation.

A single color reading may not completely describe the visual effect. Evaluation may require:

• Controlled visual comparison
• Multiple measurement angles
• Consistent application orientation
• Approved production samples
• Limits on reclaim use
• Agreement between the powder supplier, coater, and customer

Visual Appearance Inspection

Visual inspection should be performed under controlled and repeatable conditions. A part that looks acceptable under dim yellow shop lighting may look entirely different in daylight or under the customer’s lighting.

The inspection procedure should define:

• Lighting type and intensity
• Viewing distance
• Viewing angle
• Inspection time
• Background color
• Whether magnification is permitted
• Critical and noncritical surfaces
• Approved appearance standards
• Allowable defect size, quantity, and distribution

What to Look for During Visual Inspection

• Correct color and gloss
• Uniform hiding and coverage
• Consistent texture
• Orange peel
• Cratering and fisheyes
• Pinholes and outgassing defects
• Dirt, fibers, and inclusions
• Sags or excessive buildup
• Light edges, corners, and recesses
• Back-ionization marks
• Color contamination
• Rack marks and hook shadows
• Masking and plugging condition
• Damage caused during handling

Use an Approved Appearance Standard

Words such as smooth, acceptable, slight orange peel, clean, and good gloss can mean different things to different people.

An approved physical sample can provide a practical visual reference. Protect it from:

• Sunlight and ultraviolet exposure
• Heat
• Abrasion
• Chemicals
• Fingerprints and contamination
• Uncontrolled cleaning

Label the standard with its approval date, powder information, gloss range, color tolerance, and authorized use. Replace it when damage or aging makes it unreliable.

Test Results Must Be Connected

Observed Result	Possible Process Direction
Poor adhesion with acceptable cure	Investigate cleaning, pretreatment, contamination, and substrate condition
Poor solvent resistance with acceptable appearance	Investigate time at metal temperature and powder identification
Gloss varies across the part	Check film thickness, oven uniformity, application, and substrate profile
Color shifts between racks	Check cure exposure, powder batch, reclaim, contamination, and measurement conditions
Adhesion fails only at heavy film	Investigate excessive powder application and cure through the coating
Defects appear only after cure	Check outgassing, contamination, powder condition, film thickness, and oven environment
Instrument passes but visual appearance fails	The measured property may be acceptable while texture, defects, or uniformity remain unacceptable

Do Not Test Until Something Passes

Repeatedly moving the instrument or selecting a different test location until an acceptable result appears is not inspection.

A controlled retest procedure should define:

• When a retest is permitted
• How many additional tests are required
• Where the additional tests are performed
• How the original failed result is recorded
• Who evaluates and approves the final disposition

Practical Quality-Control Sequence

1. Verify the part and powder identification.
2. Review the required tests and acceptance limits.
3. Confirm that the part has cooled and conditioned.
4. Inspect color, coverage, texture, and visible defects.
5. Measure cured-film thickness at the required locations.
6. Measure gloss using the specified geometry.
7. Compare color visually and instrumentally as required.
8. Perform the approved cure-verification test.
9. Perform adhesion, hardness, impact, or flexibility tests as required.
10. Record individual results and observations.
11. Separate or hold nonconforming material.
12. Investigate the process rather than repairing the paperwork.

Quality-Control Checklist

Safety Notice

Knowledge Check

The Bottom Line

No single test proves complete powder-coating quality. Adhesion, cure, hardness, gloss, color, film thickness, and visual appearance each evaluate a different part of the finished result.

Use written procedures. Verify the instruments. Control the test conditions. Record failed results as carefully as passing ones. Most importantly, connect the test result to the production process that created it.

Quality control is not finding a number that passes. It is proving that the coating meets the requirement.