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Flash rust, adhesion failure and slow cure — what each usually indicates

Reference material, not engineering advice. Every technical claim traces to the cited standard or reference. Technically reviewed August 2026.

Published 2 August 2026 · Core Coatings technical desk

Three problems come up more than anything else in a steel workshop: rust appearing on freshly blasted steel, a coating that lets go, and a film that will not harden.

Each is a symptom, not a diagnosis. What follows is what each one usually points to and what is worth checking first. It is not a prescription for any particular job — the real cause depends on the steel, the system and the conditions on the day.

Flash rust

What it is

Flash rust is rusting that occurs on metal within minutes to a few hours after cleaning is complete. The joint SSPC and NACE waterjetting standard describes it as a light oxidation of the steel that occurs as waterjetted carbon steel dries, and notes that with the exception of stainless steel, any steel surface may show flash rust within 0.5 to 2 hours after cleaning by water — or longer, depending on environmental conditions.

The same standard grades it:

With time, flash rust changes from a yellow-brown, well-adherent, light rust to a red-brown, loosely adherent, heavy rust.

What it usually points to

Flash rust means moisture reached bare steel and stayed long enough. The three inputs are moisture, soluble salts and time, and the salts do most of the damage — sodium chloride pulls moisture out of the atmosphere at around 75 per cent relative humidity, and less abundant salts draw moisture as low as 25 to 35 per cent. As humidity rises, the corrosion rate rises sharply.

The practical signal: how fast it appears tells you which input is dominant. Rapid flash rusting may indicate salt contamination on the surface, high humidity, or both.

What to check first

• Soluble salts on the prepared surface. Tested to AS 3894.6 by the Bresle patch method. Real specifications set hard limits — one published government specification requires soluble salts below 5 µg/cm² as a hold point, and chloride contamination below 50 mg/m² for above-ground steelwork.
• Relative humidity and dew point. Below 85 per cent relative humidity, and steel surface temperature more than 3 °C above dew point.
• The blast-to-coat window. Published specifications run from as little as 1 hour in an exposed coastal environment to 8 hours in a sheltered inland non-polluted environment where the steel stays 3 °C above dew point — with 4 hours the common paint-side requirement.
• Whether the black rust was pre-washed. One specification requires a pre-wash within the previous 48 hours specifically to remove salt contamination before blasting.

Note that most high-performance systems will tolerate light flash rust and nothing worse, and the standard itself says the coating manufacturer should be consulted to establish what the candidate coating will actually tolerate.

Adhesion failure

What it usually points to

Where the coating separates tells you most of the story.

If it comes off the steel, taking the whole system with it, the issue is almost always at the substrate: contamination that was never removed, insufficient profile, soluble salts under the film, or flash rust that was coated over.

If it separates cleanly between two coats, it is an intercoat problem, and the causes are a short, well-known list: contamination between coats, exceeding the maximum overcoating interval, and glossy surfaces that were not abraded before recoating.

The recoat window is worth stating plainly because it is counter-intuitive: with most industrial coatings there is a maximum as well as a minimum time before the next coat. Leave it too long and the surface has to be abraded and cleaned before the next coat will bond. On a job that stops over a long weekend, that is a real risk.

What to check first

• Adhesion, tested properly. AS 3894.9 is the Australian site method. One published specification uses the X-cut tape test, method A of ASTM D3359, requiring an average rating of 4A or better with no single result worse than 3A, and requires rejected areas to be removed, prepared and recoated.
• The daily records. Ambient and surface conditions, recorded every 4 hours. If conditions were outside limits when the coat went on, the cause is usually in there.
• The recoat interval actually used, against what the specification and the product require.
• Preparation records — blast grade, profile, salts, dust rating.

Slow cure

What it usually points to

Two-pack coatings cure by chemical reaction, and reactions slow down when it gets cold. What is less obvious is that a slow cure is often not just slow — it can be a different, worse cure.

The clearest example is amine blush, also called amine exudate or bloom. The amine in the Part B is supposed to cross-link with the epoxy resin. Instead it reacts preferentially with more accessible compounds — moisture and atmospheric carbon dioxide — and the result migrates to the surface of the film.

Conditions that cause it:

• cold, damp application conditions
• high relative humidity
• surface temperatures at or below dew point
• inadequate induction time before application
• equipment exhaust or heating systems introducing reactive gases

It appears first as a milky off-white or pale yellow haze with an oily feel, then oxidises through yellow and tan to a dry brown film — getting progressively harder to remove.

And the consequence is the one that matters: overcoating wet blush creates adhesion failure.

Applying at low temperature slows the reaction and increases the likelihood of blush, because the amine is exposed for longer. And if the substrate falls below dew point, moisture in the air condenses on it — exactly the condition the 3 °C rule exists to prevent.

Not every slow cure is temperature. Cure also depends on getting the mix ratio right, mixing thoroughly, and observing the induction time.

What to check first

• Steel surface temperature versus dew point, not air temperature. More than 3 °C above dew point.
• Relative humidity, below 85 per cent.
• Whether the surface is blushed — the oily haze and the feel are the field indicators; field kits based on pH or carbamate detection give a quick screen.
• Mix ratio, mixing and induction time.
• For inorganic zinc silicates specifically: cure before overcoating is a measured property, not a judgement call. One specification requires a minimum resistance rating of 4 to ASTM D4752 (the MEK solvent rub test) on solvent-borne ethyl zinc silicates before any organic coating goes over the top.

The short version

What this means for you

Every one of these is decided by conditions and records, not by the tin. The same three problems keep recurring because the checks that would catch them happen before the problem appears, and only get looked for afterwards.

If any of these have shown up on your work in the last twelve months, that is one of the seven questions in the Paint Audit — and the read maps each issue you report to what it usually points to, so you have something concrete before anyone talks about product.

References

• SSPC-SP 12 / NACE No. 5, Surface Preparation and Cleaning of Metals by Waterjetting Prior to Recoating
• Corrosionpedia, 7 Things to Know About Flash Rust
• NZ Transport Agency, S9 Specification for coating steelwork on highway structures, October 2020
• KTA-Tator, Causes, Detection and Removal of Amine Blush
• PCI Magazine, Amine Blushing and Blooming of Epoxy Binder Systems in Protective Coatings
• AS 3894.9-2003, Site testing of protective coatings — Determination of adhesion
• ASTM D4752-20, Standard Practice for Measuring MEK Resistance of Ethyl Silicate (Inorganic) Zinc-Rich Primers by Solvent Rub

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