During paint spraying, edge and corner areas are prone to film defects such as edge pulling, fat edge, thick edge, and sagging. This article provides a detailed analysis of the core causes of these problems, including surface tension imbalance, differences in solvent evaporation rates, insufficient substrate surface energy, and the influence of application techniques, and introduces solutions such as wetting and leveling agents, substrate treatment, spraying techniques, and formulation optimization, applicable to automotive coatings, industrial coatings, wood coatings, and anti-corrosion coatings.
During automotive painting, industrial anti-corrosion, wood furniture, and metal coating processes, edge and corner areas are often prone to:
- edge pulling
- fat edge
- edge thickening
- bare substrate showing through
- sagging
- film buildup
These issues not only affect the appearance, but also reduce the overall quality and service life of the coating.
Many applicators believe this is a spray gun issue or a problem with the paint quality itself, but in reality, the vast majority of edge and corner defects are closely related to "surface tension equilibrium" and "edge evaporation rate".
This article systematically analyzes the formation mechanisms of edge and corner defects in spraying and common solutions in the industrial coatings industry.
I. What are edge retreating, fat edge, and thick edge?
Before formally analyzing the causes, it is necessary to first understand these common phenomena.
1. Edge pulling
Edge pulling refers to:
The paint exhibits significant retraction in edge and corner areas, resulting in:
- film thickness reduction
- partial bare substrate exposure
- Incomplete coverage
In severe cases, it will form obvious edge voids.
2. Overfill / Beading (excess coating at edges)
Fat edge typically manifests as:
The coating film at edges and corners is noticeably thicker than on flat surfaces.
This phenomenon can lead to:
- uneven gloss
- edge protrusion
- appearance imbalance
3. Edge thickening
Edge thickening is a condition where fat edge has further worsened.
When excess coating accumulates at edges and corners, it is prone to forming:
- Excessive film thickness
- sagging
- slow drying
- Increased risk of localized cracking
II. Root Cause of Edge/Corner Issues: Surface Tension Imbalance
The coating film after spraying is in a liquid fluid state.
The liquid will naturally flow from the "low surface tension region" to the "high surface tension region".
This is the key reason for the formation of edge and corner issues.
III. Why are edges and corners more prone to problems?
1. Solvent evaporation at edges and corners is faster.
Edge and corner areas typically:
- Larger surface area exposed to air
- Faster heat dissipation
- Higher solvent evaporation rate
After the solvent rapidly evaporates:
Surface tension in edge and corner areas increases rapidly.
While the solvent in the middle region evaporates more slowly, resulting in a relatively lower surface tension.
Thus, the following will be formed:
tension difference
Ultimately, this causes a change in the flow direction of the paint liquid.
IV. Why does the paint flow toward the edges and corners?
According to the laws of liquid flow:
Liquids generally flow from low-tension regions to high-tension regions.
Therefore:
- Low tension in the central area
- High tension in the edge region
The paint liquid will gradually move toward the edges.
Finally formed:
- edge buildup
- film thickness increase
- fat edge
- edge thickening
If the tension fluctuation becomes more severe, it may also lead to:
- edge retraction
- bare substrate showing through
- shrink-back
This phenomenon is particularly evident in high-gloss topcoats.
5. Mismatch of substrate surface energy is also a significant cause.
Apart from solvent evaporation issues, the condition of the substrate is also critical.
If the substrate has:
- oil stains
- dust
- release agent residue
- Surface energy too low
this will result in:
The coating fails to wet the substrate uniformly.
Thereby producing:
- local shrinkage
- cratering
- edge pulling
- poor adhesion
Especially in:
- plastic parts
- aluminum material
- galvanized sheet
- high-gloss substrate
Surface tension matching is even more important.
VI. Manifestation of Edge/Corner Issues in Different Coating Systems
1. Automotive paint
In automotive painting:
Edge and corner areas are prone to:
- fat edge
- sagging
- gloss difference
This is especially pronounced in high-solids automotive clearcoat systems.
2. Industrial anti-corrosion paint
During spraying of industrial equipment:
Edge thickening may cause:
- uneven drying
- Increased internal stress within the paint film
- Late-stage cracking
It is particularly common in the edge zones of steel structures.
3. Wood Coatings
The edges of woodwork exhibit stronger absorbency.
If application is uneven:
Prone to:
- edge darkening
- Uneven film thickness
- sandthrough
7. How to Resolve Edge Recession, Beading, and Thick Edge Issues?
A truly mature industrial coating system typically achieves this through:
- Formula optimization
- additive balance
- Construction process control
Jointly resolve these issues.
VIII. Solution 1: Add a wetting and leveling agent
Wetting and leveling agents are common coating additives.
Its main functions include:
- reduce surface tension
- improve wetting performance
- Improve leveling effect
- Reduce edge buildup
Reasonable use of leveling agents can effectively reduce:
- edge pulling
- cratering
- fat edge
- brush marks
Currently at:
- Automotive paint
- industrial coating
- UV coatings
- wood coatings
It is very widely used in.
IX. Solution 2: Enhanced Substrate Preparation
The substrate must be ensured clean before spraying.
The standard processing procedure includes:
- Degreasing
- dust removal
- sanding
- Increase surface energy
If necessary, it is also required to:
- flame treatment
- corona treatment
- plasma treatment
It is especially important for low-surface-energy materials.
X. Solution 3: Optimizing the Spray Coating Process
The construction process has a very significant impact on edge and corner quality.
Correct practices include:
- Apply thin coats multiple times on edges and corners
- Avoid applying a heavy coat in a single pass.
- Control the spray gun distance
- Control spray air pressure
- Adjust the atomization effect
This can reduce the concentrated accumulation of liquid at the edges.
XI. Solution 4: Properly Adjust Coating Viscosity
Appropriately increasing the system viscosity can reduce excessive flow of the paint.
However, higher viscosity is not always better.
If the viscosity is too high:
It may also lead to:
- poor leveling
- orange peel
- haze
Therefore, a comprehensive balance must be struck:
- flowability
- leveling
- edge coverage
12. How do modern coating formulations address edge and corner issues?
Modern industrial coating manufacturers typically achieve this by:
additive synergistic balance
Including:
- wetting agent
- leveling agent
- defoamer
- solvent system
- resin combination
Achieve overall stability control.
High-end systems will also be optimized:
- evaporation gradient
- surface tension curve
- edge wetting ability
- Film rheological properties
thereby improving edge and corner consistency.
13. How to Reduce the Rework Rate at Edges and Corners?
For spraying enterprises, edge and corner defects will directly increase:
- sanding rework
- material loss
- Labor cost
- Production cycle
Therefore, it is recommended that:
Establish a standardized spraying system
Including:
- fixed dilution ratio
- Fixed spray gun parameters
- Fixed construction temperature and humidity
- fixed flash-off time
This can significantly improve stability.
Conclusion
During paint spraying, the occurrence of edge pulling, fat edges, and thick edges is essentially the result of the combined effects of coating surface tension, evaporation rate, and substrate wettability.
Whether it is automotive paint, industrial anti-corrosion paint, or wood coating, achieving a smooth and even edge and corner finish requires:
- a rational formulation system
- precise additive balance
- scientific construction technology
- Good substrate preparation
Only through comprehensive optimization can a high-quality spraying effect with uniform film edges, stable appearance, and long-term durability be truly achieved.