What Is a Surface Profile?
A surface profile is the degree to which a surface departs from flatness. On a microscopic scale no surface is perfectly flat; there are peaks and valleys. That roughness creates the mechanical key a paint or coating needs in order to hold.
- Too smooth a surface → the paint does not key, it slides and runs
- Too rough a surface → the paint cannot cover the peaks and the thick film is left open in places
- The correct roughness → a mechanical key plus a continuous coating → long-lasting work
Ra and Rz — the Basic Difference
Ra — Average Roughness
- The average value of all the peaks and valleys
- Expressed in micrometres (µm)
- A single figure — a simple summary
- Describes the general character of the surface
- Standard measurement: a profilometer (stylus scanner)
Rz — Peak-to-Valley Height
- The difference between the highest peak and the deepest valley
- In micrometres (µm)
- The average of five sampling lengths
- Describes the extreme behaviour of the surface
- The MORE MEANINGFUL metric for paint and coatings
ISO 8503 — Three Profile Grades
ISO 8503 divides a blast-cleaned surface into three grades: fine, medium and coarse. The comparator plate carries four segments, but those are not four grades — each grade covers the range that begins at one segment and runs up to the next. The fourth segment is the upper limit of the coarse grade.
| Grade | Angular / Grit (G) | Round / Shot (S) | Typical Application |
|---|---|---|---|
| Fine | from 25 µm to 60 µm | from 25 µm to 40 µm | Thin paint, delicate alloys |
| Medium | from 60 µm to 100 µm | from 40 µm to 70 µm | Industrial paint — the most common |
| Coarse | from 100 µm to 150 µm | from 70 µm to 100 µm | Thermal spray, heavy coatings |
ISO 8503 defines two separate reference comparators, and the letters indicate the type of abrasive used: "G" represents the profile produced by grit (angular grain) abrasives, while "S" represents the profile produced by shot (round, ball-type metallic) abrasives.
- The G comparator (grit) — angular grain such as garnet, aluminium oxide or steel grit; a sharp peak-and-valley profile
- The S comparator (shot) — round grain such as steel shot or glass bead; a soft, dimpled profile
- An angular (G) profile is generally better for paint adhesion — it gives more mechanical key area
- Where grit and shot are used together the standard recommends the G comparator
- A measurement below the fine grade is reported as "finer than fine", and one above coarse as "coarser than coarse"
Matching Abrasive to Profile
| Abrasive | Mesh | Typical Rz Profile | Suitable Coating |
|---|---|---|---|
| Glass bead | 60-80 | 15-30 µm (very fine) | Decorative, visible surfaces |
| Garnet | 80 | 40-60 µm (medium) | Industrial paint |
| Garnet | 50 | 60-80 µm (thick) | Epoxy, heavy coatings |
| Aluminium oxide | 30-40 | 80-120 µm | Thermal spray |
| Steel shot | 20-30 | 90-120 µm | Structural steel, heavy duty work |
Measuring the Profile — 3 Methods
- Profilometer (stylus) — the laboratory standard, ±1 µm accuracy, expensive
- Replica tape (test tape) — the site standard, an impression measured with a micrometer
- Comparator plate (ISO 8503) — visual comparison, the fastest but subjective
In industrial workshops replica tape (Press-O-Film) is the most widely used method — a measurement in seconds, accurate enough and cheap.
The Relationship Between Paint Thickness and Profile
Paint thickness has to be chosen against the deepest valley:
- An Rz 50 µm surface + 100 µm of paint = the valleys are filled completely, an even film
- An Rz 50 µm surface + 40 µm of paint = the peaks are left exposed and corrosion starts
- The general rule: dry film thickness (DFT) = Rz × 2-3
- Applications that need thick paint (epoxy, zinc-rich primer): Rz × 3-4
Common Mistakes
- Over-roughening the surface → paint thickness rises and so does cost
- Under-roughening the surface → the paint does not key and peels years later
- The wrong abrasive → the wrong profile type (S where G was needed)
- Painting without measuring the profile → work with no guarantee of quality
- Using worn abrasive → the grain has lost its sharpness and the profile comes out wrong
- Dust or oil residue → the paint will not key, however good the profile
Industry Standards
- ISO 8501 — grade of surface preparation (Sa1, Sa2, Sa2½, Sa3 — the cleanliness level)
- ISO 8502 — measurement of surface contamination (chloride, oil, dust)
- ISO 8503 — measurement of surface profile (Rz, profile type)
- ISO 8504 — surface preparation methods
- NACE / SSPC — the American equivalents (SSPC SP6, SP10 and so on)
- EN ISO 12944 — paint systems and durability classes