In the modern painting industry, whether it is wood coatings, industrial coatings, automotive refinish paints, or architectural exterior wall paints, the spraying quality directly affects the aesthetics, durability, and market competitiveness of the final product. Among them, excessively thick paint spraying is one of the most common and easily overlooked process problems. Once the paint film is too thick, it often triggers a series of chain reactions such as sagging, slow drying, cracking, loss of gloss, and reduced adhesion. In severe cases, it may even require rework of the entire section. Therefore, systematically understanding the causes, mastering remediation methods, and preventing issues in advance during the production process are technical aspects that every coating enterprise must focus on.
1. Core Reason Analysis of Excessive Paint Spray Thickness
1. Incorrect spray gun parameter settings
The air pressure, spray pattern, and nozzle size of the spray gun determine the atomization effect.
- Low air pressure → large atomized particles → local accumulation forming a thick film
- Uneven spray pattern → prone to spitting "coarse dots" and "concentration".
Therefore, incorrect spray gun settings are one of the most common factors leading to excessive film thickness.
2. Non-standard operating technique
Unskilled painting personnel can directly cause over-thickness issues. Typical manifestations include:
- Spray distance too close (within 15 cm)
- Spray gun moving too slowly
- Overlap rate exceeds 70%
- corner angles and edges requiring repeated touch-up spraying
These operations will cause the paint to accumulate excessively in certain areas.
3. Coating viscosity too high
When viscosity is high and flowability is poor:
- poor atomization
- Coating particles that fall onto the surface do not spread easily.
- Prone to thick film, orange peel, and a sense of buildup.
This is especially noticeable when temperatures are low in winter.
4. Spraying too many coats at one time
Some spray painters, in pursuit of "faster coverage" and "brighter gloss," will apply multiple coats in quick succession without allowing sufficient flash-off time, causing the inner-layer solvents to fail to evaporate in time and forming a overall excessively thick wet film.
5. Ambient humidity and temperature not up to standard
Environmental conditions can greatly affect film formation:
- Low temperature, slow solvent evaporation → pronounced thick-film application
- High humidity makes the film difficult to dry → the surface is prone to sagging.
- Insufficient ventilation → Excessively thick particle deposition
Therefore, improper environmental control often makes thick-film coating problems more difficult to handle.
II. Typical Problems That May Be Caused by Applying Paint Too Thickly
1. Sagging, Running
The mostintuitive defect in thick coating is the appearance of obvious sag marks on the surface, which seriously affects the appearance.
2. Drying time is significantly prolonged
The thick film hinders solvent evaporation, reduces efficiency, and slows down the overall production tempo.
3. Defects such as cracking, loss of gloss, and cratering
Thick coating creates a difference in drying rates between the inner and outer layers, leading to:
- Excessive surface stress → Cracking
- Uneven solvent evaporation → loss of gloss
- Surface tension variation → craters, orange peel
4. Adhesion reduction
A thick film does not mean stronger; on the contrary, the increased internal stress within the paint film can lead to reduced adhesion.
5. Reduced service life
Thick-film coatings have an unstable film structure and are more prone to premature aging under outdoor exposure or mechanical friction.
3. Effective Remedial Methods for Paint Applied Too Thickly (Categorized by Severity)
1. Slightly too thick: just sanding + polishing
Suitable for situations where the surface is flat but the thickness is relatively large.
Tool suggestions:
- 1000–1500# wet sandpaper
- sponge sandpaper
- polishing paste
Key operating points:
- Lightly abrade to remove high spots
- Maintain uniform force
- Finally, apply a light mist coat to restore the gloss.
2. Sagging occurs: Repair with sagging knife + sanding
After the coating is completely dry:
- Lightly scrape off the sag with a drawdown knife to level it.
- Then sand with 800–1500# sandpaper.
- Finally, re-spray a thin layer.
This is the most standard process procedure for repairing sagging.
3. Overall excessively thick: must be re-sanded and reworked
If the film thickness reaches a level that affects adhesion, causes obvious orange peel, or results in unstable surface tension, then it is necessary to:
- Thoroughly sand back to the substrate using 320–600# sandpaper.
- redo substrate preparation
- Re-apply in multiple thin coats according to the standard.
Although time-consuming, this is the only solution to ensure quality.
4. Adjust the coating system
When excessive film thickness is caused by high viscosity, the following should be done:
- Increase the amount of diluent added
- Control viscosity within the process-recommended range
- Apply multiple thin-coat spraying instead of a single heavy-coat spraying.
4. How to Prevent Excessive Paint Spray Thickness at the Source (Key Focus for Enterprises)
1. Establish a standard spray gun parameter table
To be established based on the actual paint system used:
- Pressure range
- Nozzle model
- spray pattern width
- standard spray distance
Standardize procedures across all operators to improve consistency.
2. Standardized Operation Technique Training
Standardized training must be provided to spraying personnel on a quarterly basis, including:
- Maintain a spray gun distance of 15–25 cm.
- Uniform spraying speed
- Overlap rate controlled at around 50%.
- Avoid repeated touch-up spraying.
This is the most effective way to reduce issues with thick-film application.
3. Control Coating Viscosity
Measure the viscosity using a viscosity cup (Ford cup No. 4, NK-2, etc.) and record the data.
4. Strictly control flash-off time
Allow at least 5–15 minutes between coats for the solvent to fully evaporate.
5. Optimize the Spraying Environment
- Temperature: 20–30°C
- Humidity: 40%–75%
- Ventilation: Avoid direct strong blowing.
A good environment can significantly reduce the incidence of coating defects.