русский
Español
FrançaisIndustry News
Blotchy color after leather finishing is not necessarily a color-paste problem. Substrate absorbency, wetting of the finishing liquid, pigment dispersion, coating thickness and drying conditions can all cause local color differences. Improving blotchiness means analyzing substrate condition, finishing formulation and processing together.
In leather and synthetic leather production, finishing affects not only product color but also surface feel, gloss and performance. To get an even look, producers usually adjust color-paste ratios, finishing-liquid viscosity and application conditions.
Yet sometimes the finishing color has not changed, even the same batch is applied continuously, and the finished surface still shows light and dark patches, local blotches or gloss differences. That does not necessarily mean the pigment was mixed wrongly. Absorbency of the leather surface, spreading of the finishing liquid, coating thickness and the drying process can all change the final visual result. To find the cause, separate the color itself from the coating-forming process.
Common assumption versus what actually matters
Common assumption
The color paste ratio must be wrong.
Local color difference should be fixed by adjusting the color paste directly.
More wetting agent will fix uneven spreading.
Every color difference needs defoaming.
What actually matters
Check substrate absorbency, wetting, pigment dispersion, coating thickness and drying.
Check whether the substrate is uniform first: surface cleanliness, absorption state and pretreatment.
Wetting agents are not better the more you add; verify against formulation compatibility and actual application.
Not every color difference needs defoaming.
Four common causes of blotchiness
1. Uneven substrate absorbency
Natural leather is somewhat non-uniform. Fiber structure, surface state and absorbency can differ between areas, so even the same finishing liquid may penetrate, spread and stay differently. Where absorption is fast and less coating remains, color may look lighter; where more coating remains, it may look darker. For synthetic leather, differences in base fabric, surface-treatment layer and existing coating can also affect later finishing. When local color difference appears, check substrate uniformity first, including surface cleanliness, absorption and pretreatment, instead of adjusting the color paste ratio directly.
2. Uneven wetting
The finishing liquid must spread fully on the leather to form a relatively even coating. If the system does not match the surface properties of the substrate, or oils and release residues are present, spreading may be incomplete in places. The liquid may shrink in some spots and concentrate in others, ending in light and dark patches, exposed substrate or uneven gloss. Check substrate cleaning, finishing-liquid surface tension and suitability of wetting additives. Wetting agents are not better the more you add and still need verification for compatibility and application performance.
3. Unstable pigment dispersion
Even with an identical color-paste ratio, pigment that is not fully dispersed, or that flocculates and re-agglomerates during storage and conveying, can affect color uniformity after finishing. Changed particle state can make the pigment distribution uneven, affecting hiding, color strength and surface appearance. Besides checking color records, check the storage state of the paste, how evenly it was stirred and its compatibility once added to the finishing system. For hard-to-disperse pigments, further evaluate whether the dispersant matches the resin system.
4. Uneven thickness and drying speed
Spray distance, gun travel speed, coating amount and number of passes can all affect local thickness. Thick and thin areas of the same liquid may show different hiding, gloss and depth of color. Inconsistent drying speed can also affect film formation and surface state. Check whether application parameters are stable, and whether blotchiness corresponds to spray paths, edges or drying zones. Improving application uniformity and drying conditions is often more effective than changing the color paste ratio.
Which additives can be considered
Wetting agent
Helps the finishing liquid spread on the substrate; suitable for insufficient wetting or local crawling, but cannot replace necessary substrate cleaning and pretreatment.
Dispersant
Helps the pigment keep a more stable dispersion; suitable for systems with incomplete dispersion or a changed state after storage.
Leveling additive
Improves coating spreading and flatness, but you need to confirm that the blotchiness is related to coating flow and film formation.
Defoamer
Suited to bubbles or foam causing local missed coating; not every color difference needs defoaming.
Choose additives only after the cause is clear. If the main problem is absorption difference of the leather itself or uneven application thickness, adjusting additive dosage alone usually cannot give stable results.
How to troubleshoot blotchiness
Check the substrate
Compare surface state, absorbency and pretreatment between areas.
Check the finishing liquid
Verify color records, pigment dispersion, storage time and stirring before use.
Check the process
Compare spray or coating amount, speed, thickness and drying conditions.
Run controlled tests
Fix substrate, liquid and process as far as possible, change one factor at a time, and compare color, gloss and uniformity.
If possible, measure different positions with a colorimeter to tell real color differences from visual differences caused by gloss or surface texture. For natural leather, also allow for natural differences between parts of the hide, so not every appearance change is blamed on the formulation.
Frequently Asked Questions
Does blotchy leather always mean the color was mixed wrongly?
No. Even with identical color records, substrate absorbency, wetting, pigment dispersion, thickness and drying can all cause it.
Why can the same finishing liquid look lighter in some areas?
Where the leather absorbs faster, less coating stays on the surface and the color may look lighter, while areas that keep more coating may look darker.
Should I just add more wetting agent?
No. Wetting agents are not better the more you add, and cannot replace substrate cleaning and pretreatment; verify against compatibility and actual application.
How can I separate real color difference from visual difference?
Where conditions allow, measure different positions with a colorimeter to distinguish color differences from gloss or surface-texture effects.
Blotchiness can occur even when color records are fully consistent, driven by substrate absorbency, surface wetting, pigment dispersion, coating thickness and drying conditions. The key is to decide at which stage the color difference arises, then choose the matching process adjustment or additive solution. Only by analyzing substrate, formulation and application together can finishing uniformity be improved with more precision.
