First, clarify the structure: substrate → intermediate coating → color paint → clear coat, and the delamination position is divided into three categories:
① Middle coating ↔ Delamination between colors and paints
② Color paint ↔ Delamination between varnishes
③ Internal coating, intermediate coating, and substrate delamination
The core mechanism can be summarized as follows: insufficient hydrogen bonding/chemical bonding between the two interfaces, contamination of the interface, excessive cross-linking and deactivation of the surface layer, excessive interface stress, and solvent erosion and damage. Characteristics of 2K medium coating: The curing agent (isocyanate) has a short reaction activity window, and if the surface is dry but not fully reacted, or if the surface is sealed off after baking, it will directly waste the interlayer adhesion.
1、 2K intermediate coating → delamination between colored paints
1. Curing state of intermediate coating
1) Excessive baking of 2K polyurethane intermediate coating, long baking time/high PMT, complete consumption of surface NCO, depletion of hydroxyl groups, and formation of dense inert film on the surface; The subsequent color paint cannot penetrate or chemically bond, and the interface is only physically bonded. It can be opened directly by scratching or boiling.
Phenomenon: The intermediate coating is very transparent and solvent resistant, but the interlayer is poor; The longer it is left for painting, the worse the adhesion. Industry terminology: Intermediate coating surface “cover, passivation”.
2) Insufficient baking of the intermediate coating results in excessive residual NCO and small molecule solvents, causing the surface to become soft; Paint solvents are strong, causing swelling, erosion of the coating, and damage to interfacial cohesion; Alternatively, the intermediate coating may continue to cure, causing internal stress at the interface to pull apart.
Phenomenon: The adhesion is still good just after it is done, but it peels off after boiling and wet heat cycling.
3) The ‘static curing window’ of the intermediate coating was not well controlled
Short interval: The intermediate coating is still reacting, NCO is still active, the paint solvent can swell and bite, and the adhesion is good; But the window is very narrow, which is prone to pinholes, bubbles, and biting at the bottom.
Long interval: All NCO on the surface of the 2K intermediate coating is consumed, and it will also adsorb water vapor, oil stains, and silicon pollution in the air, resulting in interface deactivation. ✅ Countermeasure: There is usually an upper limit for the recoating interval in 2K medium coating, and if it exceeds the upper limit, it must be polished! Polishing is the process of breaking the inert surface layer and creating a micro occlusal surface.
2. Interface pollution
Silicone oil, leveling agent, and wax powder migrate to the surface of the intermediate coating; Excessive organic silicon leveling agent and slip agent in the middle coating float to the surface, forming a barrier layer, and the color paint cannot stick on.
Workshop dust, mold release agent, compressed air with oil, and grease on hands;
The surface of the intermediate coating absorbs moisture: NCO reacts with water vapor to generate urea, and the surface is inert.
3. Mismatch of solvent system
The solvent of the paint is too strong: strong solvents corrode and swell the intermediate coating, causing wrinkling of the interface and collapse of cohesion;
The solvent of the paint is too weak: it cannot slightly swell the surface layer of the intermediate coating, only floating on the surface without penetrating or biting, and has poor adhesion.
Ideal state: The paint solvent can slightly swell the surface of the intermediate coating without gnawing on the bottom.
4. The formula problem of the intermediate coating itself
The molecular weight and Tg of the intermediate coating resin are too high, and the surface is dense and difficult to swell;
Excessive filler, dense powder precipitation, and surface inertness;
Selection of curing agent: HDI trimer has high rigidity, dense film formation, and is more prone to interlayer differences; IPDI has relatively good flexibility and is easier to do interlayer, but the cost is higher.
2、 Delamination between paint and varnish
1. The migration of surface additives in paint forms a weak interfacial layer, which is the primary culprit between the layers of paint and varnish; Especially the color paint has added a lot of wax powder to prevent settling and wear resistance.
2. The paint is baked too dry and the surface is highly cross-linked, making it difficult for the varnish solvent to slightly swell;
3. The solvent for the varnish is too weak, resulting in insufficient lubrication and penetration; Or the solvent is too strong, causing the paint to dissolve, the color paste to float, and the interface to become disordered;
4. The thickness of the paint film is too high, and there is residual solvent inside. During the subsequent baking of the clear varnish, the solvent rushes out and opens up the interface;
5. If the paint is left for too long, the surface will absorb dust and oil stains.
Differentiation: Delamination between paint and varnish. Often, marking is when the entire piece of varnish is peeled off from the paint, leaving the paint intact and applied in the middle.
3、 Common Process and Equipment Factors
1. The film thickness gradient is unreasonable, and the lower film thickness is too thick, resulting in excessive residual solvents; The upper layer solidifies rapidly, with the solvent sealed at the interface, and bubbles and delaminates after being wet and hot. Recommended gradient: medium coat>color coat, moderate clear coat, do not make the color coat too thick.
2. Interlayer dust removal and electrostatic discharge fail due to high static electricity, causing dust to adsorb onto the surface of the paint film, forming isolation points and point like delamination.
3. The heating rate is too fast, and the upper layer dries quickly. The lower layer solvent cannot be released in time, and the interface accumulates solvent, resulting in interlayer pressure.
4. Insufficient wettability: The surface tension of the upper layer paint is higher than that of the lower layer paint film, and shrinkage is accompanied by poor local adhesion; The migration of lower level additives increases surface tension, while the upper level has poor spreading.
4、 Quick positioning testing method
1. Peel off the tape after grid cutting:
The peeling surface is clean and separated at the interface of the two layers → interface bonding issue
The paint film itself is broken, not due to interface separation → insufficient cohesion, not interlayer adhesion
1. Delamination only after boiling in water/humid heat: there is a high probability of residual solvents, interfacial stress, and post curing stress
2. Peel off only on samples that have been left for a long time before spraying: the surface is passivated and must be polished
3. Same formula, adhesion restored after polishing: confirmed to be due to surface inertness and surface sealing issues
4. Comparison of versions without silicon leveling agent: If the adhesion improves, it means that the silicon additive migrates to a weak interface
5、 Scenario based improvement plan
✅ 2K medium coating → color paint
1. Strictly follow the manufacturer’s recoating interval:
Short recoating window: Directly apply paint within the specified time
Exceeding the maximum recoating interval: must be lightly polished (P800~P1200), dust removed before spraying
2. Control PMT to avoid overcooking of intermediate coating; Do not blindly raise the baking temperature while ensuring substrate adhesion and medium resistance
3. Intermediate coating formula: Control the amount of organic silicon leveling agent and wax powder added; Priority should be given to selecting the main resin with good compatibility with the upper layer of paint
4. Solvent compounding: The color paint retains a small amount of polar solvent with a moderate volatilization rate to achieve micro swelling and no bottom biting
✅ Paint → Clear coat
1. Try to minimize the use of precipitation wax and high migration silicon additives in paint; If you want smoothness, choose an internally added, low migration wax
2. Do not bake the paint too dry and keep a suitable active interface
3. Clear varnish system polarity matching color paint, with priority given to wettability; Control the proportion of strong solvents
Post time: Oct-26-2026