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Home / News / Industry News / Steel Structure Coating First Coat Adheres Fine — Why Does the Second Coat Cause Lifting?
Industry News
Steel Structures · Industrial Coating · Multi-Coat Application
In steel structure coating, there is a typical interlayer problem: after the primer is sprayed, adhesion is normal and the surface shows no obvious abnormality — but after the second coat is applied, the first coating film develops wrinkling, softening, cracking, or even local lifting. Many people assume the second coat "has poor adhesion." But in fact, the problem does not necessarily occur in the second coat itself. A more common situation is that the solvent in the second coat produces a re-dissolution or swelling effect on the first film that has not yet fully stabilised.
1
First Coat Dry Does Not Mean Fully Stabilised
After coating is applied, the film typically goes through a process of gradual stabilisation. Surface dryness — where the surface layer has lost obvious fluidity — does not mean the internal resin has fully formed a stable structure. Especially for coatings that require oxidation, crosslinking, or a longer cure time, there can be a significant time gap between surface dry and truly ready-to-recoat state.
2
Why Does Applying the Second Coat Cause the First to Be Lifted?
When the second coat first contacts the first film, its solvent will act on the substrate layer. If the first film has already formed a stable crosslinked structure, this influence is usually limited. But if the first film has not yet fully cured, solvent entering may cause the resin to swell, even partially re-dissolve. The internal stress generated during leveling, shrinkage, and drying of the second coating then acts on the softened substrate layer — manifesting as wrinkling, lifting, and local film damage.
3
Why the Same Formulation Sometimes Lifts and Sometimes Does Not?
Recoat interval is a very important factor. If the first film has already fully cured, applying the second coat usually reduces the chance of lifting. But if work is rushed and the second coat applied before the first has reached a suitable recoat state, problems readily appear. Particularly under low temperature, high humidity, insufficient ventilation, or excessive film thickness, the solvent release and cure speed of the first film both change — even though the surface looks dry, the interior may not have reached a stable state.
4
The Solvent in the Second Coat May Also Be the Key
Different coating systems use different solvent types with different solubility power. If the solvent in the second coat has strong solubility for the first film, and the substrate layer has not yet fully cured, obvious interlayer attack is likely. This is also why some systems are normal with the first coat, but suddenly develop lifting when a different topcoat is applied. The spray technique may not have changed — the issue may be that the solvent systems of the upper and lower layers are incompatible.
5
Why Thick Coats Are More Prone to Lifting?
The thicker the first film, the more difficult solvent release and internal cure typically are. After the surface forms a dry film, internal residual solvent still needs to gradually migrate outward. If the second coat is applied immediately, it further seals the substrate layer. Internal solvent release speed decreases, bottom stabilisation time extends. Simultaneously, the solvent in the second coat may re-enter the substrate layer. Two factors compounded: substrate softens → surface wrinkles → interlayer destabilises.
6
Why Can Additives Also Affect Recoat Performance?
Additives do not usually cause lifting, but they may affect film drying, wetting, leveling, and final film formation state. For example, some systems raise film open time to improve leveling, but if cure speed is not matched, the time for the film to reach a stable state may be extended. If additives in the second coat have poor compatibility with the substrate, interlayer problems may also be amplified. Therefore, optimising recoat performance requires considering the overall matching of resin system, solvent system, cure speed, and additives together.
7
How to Diagnose Lifting When It Occurs?
If steel structure coating shows first coat normal, second coat lifting, check in these directions: first confirm actual first coat dry degree (not just whether the surface is tack-free); then check actual film thickness (thick film significantly extends internal cure time); then compare the solvent system and solubility of the second coat, confirming whether obvious interlayer dissolution exists; then check application environment including temperature, humidity, ventilation, and actual recoat interval. If the problem appears immediately after switching the second coat material, interlayer compatibility needs focused attention.
8
How to Improve Second-Coat Lifting?
The approach is not simply adding an adhesion promoter. More rational thinking involves several simultaneous adjustments: appropriately extend the recoat interval to ensure the first film reaches a stable state; control single-coat film thickness to avoid thick substrate film causing slow internal cure; optimise the solvent matching between upper and lower coatings to reduce the second coat's attack on the substrate; adjust the resin and cure system to improve the early stability of the first film; then if needed combine with the additive system to optimise wetting, leveling, and film formation state.
Summary
Steel structure coating first coat adhesion normal, second coat lifting — this is not necessarily that the second coat has poor adhesion. The more common cause is that the first film has not yet reached a sufficiently stable state, and the solvent in the second coat entering the substrate layer causes swelling or re-dissolution, ultimately causing interlayer film destabilisation. Therefore, when encountering lifting problems, not only "can the paint hold" needs to be checked — substrate cure degree, recoat interval, film thickness, solvent system, and the compatibility between upper and lower coatings must also be considered simultaneously.
Frequently Asked Questions
How can the ready-to-recoat state of a primer be assessed more reliably than the tack-free test?
A solvent rub test provides a more useful assessment: rubbing the cured primer surface firmly with a cloth wetted with the topcoat's solvent and observing whether the primer softens or lifts indicates whether the primer has sufficient solvent resistance to withstand the topcoat application. A scratch hardness test on the primer surface at defined intervals also indicates the progression of cure — significant increase in hardness over time indicates cure is proceeding, and the recoat interval should be set to a point where hardness has substantially stabilised.
Is lifting more likely with strong solvents like aromatics than with aliphatic solvents?
Yes. Aromatic solvents (toluene, xylene) have significantly higher solubility power for most alkyd, epoxy, and polyurethane resins than aliphatic solvents (mineral spirits, VM&P naphtha). A topcoat formulated with high aromatic content will have a greater tendency to attack an incompletely cured primer than one based on slower, less aggressive aliphatic solvents. Formulating topcoats with a slower, less aggressive solvent blend, or selecting a water-reducible topcoat, can reduce interlayer solvent attack when recoat intervals cannot be extended.
