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Pencil Hardness Test (Wolff–Wilborn): How to Measure Coating Hardness
Pencil Hardness Test (Wolff–Wilborn): How to Measure Coating Hardness
Updated: September 23, 2025 · Reading time: ~6 minutes
The Wolff–Wilborn pencil hardness test (referenced in ASTM D3363 and ISO 15184) is a fast way to check
whether a coating has cured sufficiently. If a coating does not cure correctly, performance can suffer
(surface flaws, poor adhesion, early failure). In general, harder coatings indicate a more complete cure and better durability.
Pencils of known hardness are pushed across the coating at a consistent angle and force. Increase hardness until a
visible scratch occurs (or start hard and go softer until the film is not scratched).
Report the hardest grade that does not scratch (and/or the softest grade that does), per your spec.
Note: This is a surface, comparative, and slightly destructive test.
Use sacrificial test panels or inconspicuous areas when possible.
Important: Before testing, review product-specific instructions
and the applicable standards to ensure you are using the correct
method, conditioning, angle, load, stroke length, and reporting criteria for your application.
Equipment
Set of 14 pencils (6B to 6H), prepared to a flat, square edge.
Two pencil sharpeners and abrasive paper for proper preparation.
Guided tester (e.g., Elcometer 501) for repeatability.
Clean, dry, fully cured coated panel or representative test area.
Step-by-Step Procedure
Verify cure & clean: Ensure the coating is cured per product instructions. Remove dust/debris.
Prepare pencils: Sharpen, then rub the tip on abrasive paper to form a flat, square face.
Set angle & load: Hold at ~45° to the surface, or use a guided tester preset per the method.
Stroke: Push forward about 6–7 mm (¼ in) with steady speed and force; make a fresh line each time.
Progress grades: Start near HB and move B → HB → H until a clear scratch occurs;
or reverse from hard to soft until no scratch occurs.
Inspect: Distinguish a scratch (coating cut) from graphite smear. Use good light/magnification.
Record: Report the no-scratch grade, the scratch grade, or both (ASTM D3363 / ISO 15184 as applicable).
Pencil Grades (Soft → Hard)
Order
Notes
6B, 5B, 4B, 3B, 2B, B
Softer cores; smear easily; less likely to cut the film.
HB
Middle grade; common starting point.
F (optional)
Between HB and H.
H, 2H, 3H, 4H, 5H, 6H
Harder cores; increasingly likely to scratch or gouge the coating.
