ASTM D610 and SSPC-VIS 2: assessing the degree of rusting on painted steel

ASTM D610, published jointly with SSPC-VIS 2, rates the degree of rusting on painted steel on a grade scale from 10 (no or almost no rust) to 0 (surface almost fully rusted), followed by a letter describing the distribution: S for spot rusting, G for general rusting and P for pinpoint rusting. The result, for example "6-G", supports an objective decision between spot repair, partial repainting with an overcoat and full repainting after blasting.

Without a shared method, two inspectors can describe the same structure as "a bit rusty" or "needs a full redo". ASTM D610 gives owners, inspectors and contractors a common language. This guide is written for maintenance and project leads who manage painted steel structures, tanks or equipment in Quebec and Ontario and need to justify a maintenance painting strategy.

What does ASTM D610 measure?

ASTM D610 measures the share of a painted surface where rust is visible, and how that rust is distributed. The standard compares the observed surface with reference figures and photographs; SSPC-VIS 2 is its visual component, published jointly.

The method covers rust that breaks through the film or shows beneath it. It does not measure remaining coating thickness, adhesion or blistering, which are covered by other tests. Nor does it say why the rust appeared: root-cause diagnosis is a separate step, as explained in our article on coating failure modes.

The ISO counterpart is ISO 4628-3, which expresses the degree of rusting on a different Ri scale. The specification states which one applies; the two do not always convert one to one.

How do you read the grade scale?

The grade runs from 10, the best condition, to 0, the worst. Each grade matches a range of rusted surface percentage defined in the standard; the grade is read by comparing the area with the reference figures, not by guessing a percentage by eye.

• High grades. Rare or very limited rust: the coating still protects most of the surface.
• Intermediate grades. Rust on a noticeable part of the surface, often concentrated at edges, welds and connections.
• Low grades. Widespread rust: the system has lost its protective function on much of the structure.

The rating always includes the distribution type. "7-S" and "7-P" have the same rusted area but call for different repair strategies.

What is the difference between spot, general and pinpoint rust?

The three types describe how rust is spread out, and that distribution drives the repair directly. The table sums up their features.

Pinpoint rust is often misleading: the rusted area looks small, but it signals film permeability across the whole zone. Touching up only the visible specks is usually not enough.

How do you run a reliable survey?

A reliable survey splits the structure into uniform zones, rates each one separately and documents everything with located photos. A single average rating for a whole structure hides the critical areas.

1. Split. Zones by exposure (roof, north face, splash zone), by element (beams, bracing, connections) or by tank surface.
2. Observe at a useful distance. Constant distance and lighting, dry surface, dirt removed if it hides rust.
3. Compare. Grade and type against the reference figures of the standard.
4. Flag special areas. Edges, welds, bolting and column bases rated separately.
5. Complete. Dry film thickness, adhesion of the existing system, blistering and any contamination.

The additional tests are described in our article on coating inspection: DFT, adhesion and holiday testing. Without adhesion data, the rust grade is not enough to decide on an overcoat.

Timing matters too. A survey done a few months before the budget cycle or a shutdown leaves time to prepare the specification, check for lead and book the work window. Repeating the survey with the same zoning at each inspection makes the grades comparable from year to year, and photos taken from the same points show the change at a glance for managers who will not visit the site.

How do you use the grade to choose the repair?

Grade and type point to three strategies: spot repair, partial repainting with an overcoat, or full repainting after blasting. The decision thresholds are not set by ASTM D610: they come from the owner's maintenance program or specification, which accounts for exposure and the target service life.

Overcoating requires a compatibility check between the old and new systems, as explained in our article on painting over old coatings. Lead-based paint also changes the strategy, since blasting then requires containment.

Why does early action cost less?

Acting while the grade is still high costs less, because spot repair or overcoating avoids full blasting, which is often the heaviest part of a painting job. Every grade lost narrows the options.

Tracking the grade over time supports planning: a zone rated at each inspection shows its rate of deterioration and signals the right time to act. This logic ties into the total cost of ownership of a coating. The target durability of the replacement system is then defined under ISO 12944.

Who performs the assessment and what goes in the report?

The assessment is performed by a coating inspector who knows the standard and the reference figures, ideally independent of the contractor who will do the work. The report must let a third party find each zone and understand the recommendation.

A useful report contains:

• the zoning plan and zone locations;
• the grade and type of each zone, with photos;
• supporting measurements (thickness, adhesion, blistering);
• any lead or other contaminants found;
• the recommended strategy per zone and the matching preparation.

Our coating inspection service produces this kind of survey, which then forms the basis of a steel structure painting scope and the choice of protective coatings.

Frequently asked questions

What does a rating like "6-G" mean?

The number is the grade on the ASTM D610 scale, from 10 to 0. The letter is the distribution type: here G, for general rust spread over the surface.

Are ASTM D610 and SSPC-VIS 2 two different standards?

They are published jointly and describe the same method. SSPC-VIS 2 refers to the visual reference component; specifications cite one, the other or both.

Is the rust grade enough to decide on an overcoat?

No. You also need the adhesion and thickness of the existing system, and a compatibility check for the new product. A system with little rust but poor adhesion is not overcoated.

Should edges and welds be rated separately?

Yes. They often rust first and can justify a targeted repair even when the rest of the surface is in good condition.

Need to assess a painted structure or tank before budgeting its restoration? Our coating inspection service performs the ASTM D610 survey and recommends a strategy by zone; our crews and qualified subcontractors then carry out the work, across Quebec and Ontario. Send photos, drawings and painting history through the request a quote form: pricing on consultation, reply within 24 business hours.