A crew that doubles the coat to save a pass produces a film that cracks in polygons. The mechanism is solvent loss, shrinkage and a skin that forms too early.
A tank lining is applied in one heavy pass instead of two, because the second pass would mean waiting overnight. The film looks perfect at handover. Three weeks later the surface is covered in a network of cracks forming irregular polygons, and in places the cracks reach the steel.
What actually happens inside the film
A solvent-borne coating loses volume as it dries. In a normal thin coat, that loss is uniform through the thickness and the film simply settles. In an over-thick coat, it cannot.
- Solvent leaves from the surface first. The top of the film skins over while the lower part is still wet and still contains most of its solvent.
- The skin sets before the bulk has finished shrinking. When the solvent finally diffuses out from below, the film wants to contract but the surface is already rigid.
- The result is tensile stress in the plane of the film. The coating relieves it by cracking, and because the stress is biaxial and uniform, the cracks form a polygonal network rather than a single line.
- Each crack is a path to the substrate. In a barrier coating, that is the whole function defeated in one defect.
The conditions that make it certain
- Exceeding the maximum dry film thickness per coat. Every product data sheet states a maximum. It is a process limit, not a marketing range.
- High ambient temperature and strong air movement. Fast surface drying builds the skin sooner and makes the gradient steeper. A hot day with fans on a freshly coated tank is the classic combination.
- Inorganic zinc silicate. This is the most thickness-sensitive system in common use. Above its recommended range it cracks readily, and the cracks are difficult to repair because the film is porous by design.
- Excess thinner. Thinner increases shrinkage because there is more solvent to lose, so an over-thinned and over-thick coat is the worst combination.
- Cold substrate with warm air. The surface skins while the film below stays soft, which is the same gradient in reverse.
Mud cracking is not the same as ageing cracks
The vocabulary matters when the report is written.
- Mud cracking appears during or shortly after application. Irregular polygons, often with lifted edges, and it follows the application pattern.
- Ageing cracks develop over years. They are finer, usually associated with chalking, loss of elasticity and UV exposure, and they are assessed by the cracking standards.
- The distinction decides the remedy. Mud cracking is an application defect to be removed and reapplied. Ageing cracks are a condition assessment and a maintenance decision.
How deep does it go
This is the first question after a crack is found, and it changes the repair completely.
- Cracks confined to the topcoat can often be abraded back and recoated.
- Cracks reaching the primer or the steel require removal of the film in the affected area. A crack that reaches steel is a corrosion initiation site, and painting over it seals nothing.
- Check by inspection, not assumption. A visual assessment plus a penetrant or a microscopic cross-section at a few locations gives the depth. Assuming surface-only cracks is how a small repair becomes a tank relining.
The repair path
- Establish the boundary. Mark the cracked area and extend the repair beyond the visible cracks, because the stress field extends past them.
- Remove to sound coating. Grind or blast back to a sound edge with a feathered taper, not a vertical step.
- Reapply within the thickness limits, in two passes if necessary, respecting the recoat window between them.
- Verify adhesion at the repair boundary. The interface between old and new film is where the next failure will start.
Prevention is a schedule decision
- Specify two coats where two coats are needed. The maximum per coat exists because the product cannot be applied thicker without cracking, and no amount of skill changes that.
- Control the drying rate. Moderate ventilation instead of strong air movement, and shading or cooling where the sun is directly on the steel.
- Measure wet film as you go. A wet film comb during application catches the problem before it cures. A dry film gauge catches it after, when the only remedy is removal.
- Respect the recoat window. Applying the second coat too early traps solvent; too late requires abrading the surface. Both are cheaper than removing a cracked film.
The economics that cause it
Nobody applies a coat too thick on purpose. It happens because a second pass costs a shift, a scaffolding move or a cure delay, and one heavy pass appears to deliver the same nominal thickness for less labour.
It does deliver the thickness. It does not deliver the film. The protection in a coating is a continuous barrier of the right thickness, and a cracked film of twice the thickness has less barrier than an intact film of half of it.