First, distinguish the phenomenon: biting the bottom: the solvent/component of the topcoat swells, dissolves, peels, bulges, and directly damages the adhesion of the bottom paint film; Wrinkle: The surface of the paint film first forms a film cover, and the internal solvent cannot escape, causing internal expansion. The surface is lifted up to form wrinkles, ripples, and orange peel like folds, which may not necessarily cause delamination. The two often occur together, but the mechanisms are different, and many sites occur simultaneously.
1、 Bite the bottom
1. Solvent matching issue
The bottom layer has poor solvent resistance, and the solvent solubility of the topcoat is too strong
The primer/old paint is thermoplastic acrylic, single component NC nitro paint, chlorinated rubber, and ordinary thermoplastic alkyd; Directly spray strong solvent type 2KPU on top, the solvent penetrates into the bottom layer, swells and dissolves the bottom layer, and directly bites the bottom.
Both are PU, but the bottom layer has insufficient curing and the aging time is too short: the bottom layer has not been fully cross-linked, and the solvent resistance is poor. The top is sprayed with colored paint, and the solvent bites when it is washed.
Key point: The PCDL type 2KPU itself has good solvent resistance, but its A component solvent system has strong solubility, and it still bites when sprayed on immature primer.
2. Insufficient maturation at the bottom layer
After the primer/second coat is sprayed, the flash drying time is insufficient, the curing time is too short, and there is a large amount of solvent residue and unreacted active groups in the bottom layer, resulting in low cross-linking density. When the upper layer of paint is sprayed, the solvent penetrates into it → the bottom layer swells → biting, layering, and bubbling. For example, after spraying the primer for 30 minutes, the colored paint is applied and the surface appears to be dry, but the interior has not yet cured.
3. The bottom layer is polished and damaged, or excessively polished
Polishing removes the dense cross-linked layer on the surface, exposing the insufficiently cross-linked bottom layer inside, increasing the number of pores and making it easier for the topcoat solvent to penetrate, resulting in a significant increase in the probability of bottoming out.
4. The coating system is not compatible
The primer is epoxy, and the topcoat is made of strong solvent PU, which works fine in the short term; Partial epoxy curing is incomplete and may also be swollen by PU solvents;
Mixing resin systems from different manufacturers without conducting interlayer matching tests;
Unknown type of old coating, apply 2K color paint directly.
5. Spray too thick, wet touch wet overlapping spray
One thick spray, with a large amount of solvent for the topcoat and a long residence time, continuously soaking the bottom layer; Wet touch wet superposition, solvent continues to penetrate inward.
6. Curing agent/R value issue
The bottom R value is low, the cross-linking density is insufficient, the solvent resistance is poor, the solvent resistance is weak, and it is easily swollen by the upper solvent.
Typical characteristics of biting the bottom: peeling, softening of the bottom layer after lifting, melting sensation, interlayer peeling, usually accompanied by interlayer adhesion and scrapping.
2、 Wrinkles and wrinkles
Core mechanism: rapid film formation and sealing on the surface, hindered evaporation of internal solvents, vaporization and expansion of internal solvents, causing wrinkles on the already solidified surface. Not necessarily biting the bottom, even if the same paint is sprayed too thick, it will wrinkle.
1. Imbalance of solvent evaporation gradient
There are too many fast solvents in the formula: volatile solvents such as ethyl ester and acetone can cause the surface layer to harden rapidly upon contact with air; And the slow solvent is still trapped inside the paint film. The internal solvent is heated/continues to evaporate, causing volume expansion. The surface has already hardened and cannot stretch, resulting in wrinkling. The viscosity of the PCDL system is relatively high, and many people tend to strengthen the solvent in order to reduce viscosity, which makes this problem more likely to occur.
2. Too much catalyst and too fast curing reaction
The amount of tin/bismuth catalyst added is too high, and the NCO-OH reaction on the surface of 2KPU paint is extremely fast, quickly forming a hard shell. The internal reaction is slow, and the solvent is trapped inside → wrinkling.
This is particularly prominent in high solid PCDL systems: the resin itself has high viscosity, the catalyst is slightly excessive, and there is a huge time difference between surface and actual drying.
3. Single film thickness too high
The wet film is too thick, the total amount of solvent is large, the evaporation path is long, the surface is covered first, and the internal solvent cannot come out.
4. The ambient temperature is too high, and hot air is blowing directly onto the surface
High temperature or strong wind blowing directly on the paint surface: rapid crust formation on the surface, inability to release internal solvents, and wrinkling under internal pressure; Conversely: low temperature and high humidity, slow solvent evaporation, generally less prone to wrinkling, more prone to whitening and pinholes.
5. Characteristics of resin itself
PCDL、 Polyester polyol soft segment resin has good surface toughness after film formation. Once lifted by internal solvents, it is easy to form permanent wrinkles; The hard acrylic system is relatively less prone to wrinkling, but more prone to cracking.
6. Incorrect selection of diluent
Used quick drying diluent and quick drying banana water, not the PU diluent provided by the manufacturer; Diluents have strong solubility and evaporate too quickly.
Typical characteristics of wrinkling: surface ripples, wrinkles, no delamination or peeling of the paint film, interlayer adhesion still present, only surface morphology damage; Severe cases may be accompanied by biting the bottom.
3、 Quick distinction: Is it biting the bottom? Wrinkle? Or are they both concurrent
Simple wrinkling: The wrinkles are inside the same layer of paint and cannot be peeled off, with normal interlayer adhesion;
Simple biting on the bottom: The paint film is bubbling and can be torn off, the bottom layer is soft and sticky, and there are no obvious ripples or folds;
Concurrent: Surface wrinkling and delamination upon tearing, with the solvent both dissolving into the bottom layer and exerting pressure within it.
4、 2KPU color paint rectification plan
✅ Regarding biting the bottom
1. Compulsory bottom layer maintenance: The primer should be matured according to the specifications, not just based on the surface drying; If necessary, create an isolation and sealing layer to separate the bottom layer that is not resistant to solvents;
2. Supporting experiment: First, conduct a small-scale test of the new system, spray primer → apply paint at different maturation times, and test the biting bottom;
3. Moderate polishing: Gently polish, do not completely wear through the bottom layer; Dust and oil removal after polishing;
4. Avoid thick spraying and wet contact, and control the thickness of a single wet film;
5. Ensure sufficient cross-linking density at the bottom layer, and the R value should not be too low.
✅ Regarding wrinkling
1. Diluent adjustment: Reduce ultra fast volatile solvents, mix with medium and slow volatile solvents, and flatten the volatile gradient; Avoid using quick drying diluents alone;
2. Reduce catalyst dosage, prioritize delayed bismuth catalyst, lower surface drying speed, and allow simultaneous solidification of surface and interior;
3. Control film thickness, spray multiple layers thinly, do not spray thick at once;
4. Avoid high-temperature spraying and direct hot air blowing on the paint surface;
5. In terms of formula: The PCDL system can be combined with some high Tg acrylic polyols to improve the synchronicity of film formation.
Post time: Oct-27-2026