Slight rusting, moderate blistering, some chalking: three phrases that mean nothing in a warranty dispute. How the rating scale turns them into numbers.
An inspection report says the coating shows moderate rusting and slight blistering, and the client asks whether that is acceptable. Nobody in the room can answer, because the words do not correspond to anything measurable. Two inspectors looking at the same panel will disagree about what moderate means, and a year from now nobody will remember which panel they were standing in front of.
What the standard actually does
ISO 4628 is a series of documents that classify coating defects by three parameters: the quantity of the defect, its size, and the intensity of a change in appearance.
- Quantity is rated 0 to 5. Zero means none, five means dense. The scale is defined by reference photographs, not by adjectives.
- Size is rated 1 to 5 for defects that have a measurable dimension, such as blisters or cracks.
- The result is written as a pair, for example 3(S2): quantity 3, size 2. Separating quantity from size is the whole point, because four large blisters and fifty small ones are different problems.
- The series is split by defect type. Part 2 covers blistering, part 3 rusting on a Ri0 to Ri5 scale, part 4 cracking, part 5 flaking, part 6 chalking by tape method, part 7 velvet-like appearance, part 8 delamination and corrosion near a scribe, part 10 filiform corrosion.
Why rusting has its own scale
Rusting is rated Ri0 to Ri5 rather than on the quantity and size pair, because rust spreads differently from a blister and the percentage of surface area affected is more useful than a count.
- Ri0 means no visible rust. Ri1 is a few tiny spots, and Ri5 is extensive rusting across most of the surface.
- The intermediate grades correspond to percentage bands of the assessed area. This makes the rating reproducible between inspectors, provided they assess the same area.
- The assessed area has to be stated. A rating of Ri3 on one square metre of a 5000 square metre structure is an observation, not a condition assessment. Define the reference area before inspection starts.
The four ways this gets misused
- Quoting a photograph instead of a standard. A photo shows what one defect looked like under one light. The reference photographs in the standard are the scale; the project photos illustrate it.
- Rating from a distance. Blistering is assessed by size, and size requires close inspection. A rating taken from a drone image is an indication, not a rating.
- Rating a dirty surface. Chalking, staining and dirt all read as a change in appearance. Clean the area with a soft brush and water before assessing, and let it dry.
- Rating without recording the standard and the date. A deterioration rate is what matters for a maintenance decision. One rating without a baseline tells you the condition; two ratings a year apart tell you the trend.
Chalking deserves separate attention
Part 6 describes a tape method: apply a defined adhesive tape to the surface, remove it, and compare the amount of pigment transferred against a reference scale.
- Chalking is normal for a topcoat in service. A urethane or epoxy topcoat exposed to UV will chalk. The question is how fast, and whether the pigment left is only surface pigment or the film is thinning.
- Chalking is not the same as chalking away. A coating that chalks has a surface layer breaking down. A coating that has lost measurable thickness at the same time is reaching the end of its life, and the thickness measurement is the one that matters.
- Dark colours read differently. A chalked dark topcoat shows a light haze that exaggerates the apparent severity. Compare like with like, and record the original colour.
Turning ratings into a decision
A rating only has meaning against a specification. The useful question is not whether Ri3 is bad, but whether Ri3 at year eight is consistent with a fifteen-year durability expectation.
- Set the acceptance criteria before the coating is applied. For a high-durability system in a corrosive category, an inspection at year five should specify the maximum acceptable rating per defect type.
- Track the rate, not the absolute value. Going from Ri0 to Ri1 in five years is a different asset from going from Ri2 to Ri5 in two.
- Correlate with thickness and adhesion. A blister at Ri1 with a sound film and full thickness may be a local defect. The same blister with 60 percent of the specified thickness underneath is a system problem, and the rating alone would have hidden it.
- Photograph with the reference standard in frame when the report will be used commercially. It resolves arguments that no amount of description can.
What a good inspection record contains
- The standard and part number used, and the revision year.
- The reference area assessed, and its location on the structure.
- Date, ambient conditions, surface temperature, and whether the surface was cleaned and dried.
- Ratings per defect type, written in the standard notation, plus the thickness readings from the same area.
- Photographs with a scale and a stated distance, plus the previous inspection's ratings for comparison.
None of this makes a coating last longer. It makes the decision about what to do next an engineering decision rather than a difference of opinion, which on a large structure is where most of the money is spent or saved.