Effects of Wet‑Spraying and Dry‑Spraying on Water‑Based Paint Performance
Effects of Wet‑Spraying and Dry‑Spraying on Water‑Based Paint Performance
Wet‑spraying: Paint droplets reach the workpiece in a liquid wet state, allowing sufficient time for flow and emulsion particle coalescence to form a continuous paint film.
Dry‑spraying: A large amount of moisture evaporates from paint droplets during flight, which land on the surface as semi‑dry or powdery particles with barely any flow or coalescence.
Water‑based paints are more prone to dry‑spraying than solvent‑borne paints. Owing to the high latent heat of vaporization of water, dry‑spray defects readily occur under high‑temperature, high‑wind conditions, excessive gun‑to‑part distance or high air pressure.
1. Appearance, Gloss and Color (Marked Differences between Solid‑Color and Metallic Paints)
Wet‑spraying
• Sufficient paint film levelling, high full gloss and slight orange‑peel effect.
• Solid‑color paints feature deep color and good hiding power. For metallic/pearlescent paints, aluminum flakes align in an orderly manner for stable hue.
• Dense and smooth surface.
Topcoats and clear coats shall be mainly applied by wet‑spraying.
Dry‑spraying
• Sand‑like or powdery surface, drastically reduced gloss, haze and loss of gloss.
• For metallic paints: disorderly, upright aluminum flakes, lighter color, mottling, abnormal sparkle of metallic particles and severe color difference.
• In severe cases, powder rubs off by touch. If clear coat is applied directly over a dry‑sprayed layer, particles get trapped inside the paint film, leaving permanent surface grain that cannot be removed by polishing.
2. Film Thickness and Hiding Power
• Wet‑spraying: Thicker single‑pass film and good hiding power. Risks: excessive wetness causes sagging and runs. Over‑wet application of water‑based paint leads to fast surface drying while water remains trapped inside, resulting in blistering, pinholes and solvent pop in later service.
• Dry‑spraying: Thin single‑pass film and poor hiding power requiring multiple overlay passes. The paint film consists of stacked particles rather than a continuous dense layer; the nominal film thickness is falsely high while the actual effective film thickness is low.
3. Adhesion and Inter‑layer Performance (Key Risk Points)
Wet‑spraying
• Adequate substrate wetting delivers good adhesion to the base material.
• For wet‑on‑wet processes (re‑spray after flash‑off), inter‑layer penetration yields excellent inter‑layer adhesion.
• Risks: excessive wet film or insufficient flash‑off time before subsequent coats trap residual moisture, leading to blistering, bubbling and inter‑layer delamination after baking (typical failure mode for water‑based paints).
Dry‑spraying (Major Hidden Hazard)
1. Dry‑spray powder particles accumulate on the surface with poor cohesion between particles, forming an inherently weak interface layer.
2. If re‑spraying or clear‑coating is performed without prior sanding, the overlying paint cannot fully dissolve and merge dry‑spray particles. Inter‑layer adhesion drops sharply, causing large‑area peeling and delamination in later service.
3. Surfaces with dry‑spray deposits must be thoroughly sanded before subsequent coating; over‑coating directly is prohibited.
Practical note: Dry‑spraying may be adopted for the first thin mist coat for partial hiding, yet it shall not be used for intermediate coats or topcoats.
4. Water Resistance, Salt‑Spray Resistance and Chemical Resistance
• Properly executed wet‑spraying (without over‑application) forms a continuous dense paint film with qualified water resistance, salt‑spray resistance and scrub resistance.
• Dry‑sprayed paint films contain abundant micropores among particles. Water and salt‑spray media easily penetrate, significantly degrading water resistance, corrosion resistance and chemical resistance, and increasing risks of blistering and rusting.
5. Drying, Curing and Defects
Wet‑spraying
Advantage: continuous film formation.
Potential defects: sagging, solvent pop, pinholes and blisters (moisture trapped within film), mostly induced by over‑application and inadequate flash‑off time.
Dry‑spraying
Advantage: no sagging.
Potential defects: rough texture, chalking and gloss loss; secondary pinholes and grain defects easily occur upon subsequent clear‑coating.
Unintentional dry‑spraying frequently occurs for water‑based coatings under high‑temperature, low‑humidity conditions with strong exhaust ventilation.
6. Comparison of Application Parameters for Air‑Atomized Water‑Based Spraying
|
Item |
Wet‑spraying (Standard for Topcoat) |
Dry‑spraying (Defect / Thin Mist Priming) |
|
Gun‑to‑part Distance |
15‑20 cm |
＞25 cm (Excessive) |
|
Atomizing Air Pressure |
Relatively low, 0.2‑0.3 MPa |
Excessively high pressure |
|
Paint Output |
Moderate to high |
Low paint output, fast gun travel |
|
Paint Droplet Condition |
Wet upon reaching workpiece |
Loses moisture in flight, forms semi‑dry powder particles |
|
Application Scope |
Topcoat, clear coat, wet‑on‑wet priming |
Only for initial thin mist priming; not for final coats |
7. On‑site Practical Recommendations
1. Topcoats and clear coats must be applied by wet‑spraying to achieve proper film formation; dry‑spray application is strictly forbidden.
2. Recommended process: thin dry‑spray mist coat for sealing & partial hiding → sanding → two wet‑spray topcoat passes, balancing hiding performance and surface quality.
3. Once obvious dry‑spray defects occur, the powdery rough layer must be completely removed by sanding before further coating. Direct over‑coating is prohibited.
4. Prevention of dry‑spraying for water‑based paint: moderately reduce atomizing pressure, avoid excessive gun‑to‑part distance; add slow‑down additives in high‑temperature environments and prevent direct airflow blowing onto workpieces.
术语对照表(涂装行业标准)
