What it looks like
Dry spray feels like fine sandpaper. The surface is matte, uneven and often lighter in colour than the correct film. Under magnification, it is made of separate droplets rather than a fused layer. It is common at the edges of the spray fan, on areas reached at an angle, on the far side of structural members and in overspray zones next to the area being coated.
Why it is a problem
- Porosity. Unfused particles leave paths for moisture and ions to reach the steel, which a holiday test or early rusting may reveal.
- Weak cohesion. The layer is mechanically weak and can be rubbed or brushed off.
- Intercoat adhesion failure. The next coat bonds to loose particles rather than to a sound film, so it can delaminate.
- Misleading film thickness. A rough, porous film can give acceptable thickness readings while offering little protection.
- Trapped contaminants. Dust and overspray become embedded in the surface.
Zinc-rich primers and fast-drying coatings are especially sensitive, and dry spray in a zinc-rich primer can lead to mud-cracking or poor bond of the topcoat.
Main causes and corrections
| Cause | Correction |
|---|---|
| Gun held too far from the surface | Keep the distance recommended for the equipment and product |
| Gun angled instead of perpendicular | Hold the gun square to the surface, work details separately |
| High air, steel or material temperature | Adjust working hours, shade or cool the work, use the solvent grade allowed by the data sheet |
| Wind or strong ventilation across the work | Shield the area, adjust airflow direction |
| Excessive atomizing pressure or wrong tip size | Set equipment as the data sheet and equipment guide recommend |
| Overspray falling on coated areas | Plan the spray sequence, protect finished areas |
How it is repaired
Light dry spray can sometimes be removed by sanding or brushing off the loose particles, followed by cleaning before the next coat. Heavier dry spray usually requires abrading the affected area and recoating within the recoat window. Some manufacturers allow a mist coat or a specific procedure; the data sheet or the manufacturer's representative defines the acceptable repair. Application practice is covered by SSPC-PA 1.
Our coating inspection service looks for dry spray before each coat is accepted.