Summary: Refinish 2K polyurethane clearcoat crosslinks via an isocyanate hardener, balancing hardness, weather and chemical resistance. Repair quality hinges on mix ratio, flash time, temperature window and spray parameters.
Mechanism
Base resin (OH acrylic) plus HDI-trimer hardener (NCO) adduct-cure to a dense film; performance depends on the OH:NCO ratio.
Key data
- Common mix ~2:1 clear:hardener by volume; deviations shift cure and properties.
- Flash (before set) is typically a few minutes; coats need flash to avoid blush/sag/bubbles.
- Below ~15°C cure slows and blush risks rise; use low-temp/fast hardener and warm the booth.
- Film thickness often ~40-60 µm; too thick causes sag and solvent bubbling.
Selection by type
| System | Feature | Use |
|---|---|---|
| Standard 2K PU | Balanced gloss/weather | General refinish |
| High-solids | Lower VOC, thicker coats | Eco / build |
| Low-temp cure | Cures cold | Winter / cold booth |
Defects & fixes
- Blush/haze: too-fast dry or high humidity — dehumidify, slow diluent, control flash.
- Orange peel / loss of gloss: insufficient flash or wrong viscosity.
- Lift/peel at recoat: exceed maximum recoat window — abrade first.
FAQ
Why does 2K cure in the cold?
A more reactive or catalysed hardener still crosslinks at low temperature; over-accelerating hurts flow.
What if the mix ratio is off?
Unbalanced OH:NCO gives under-cure (soft, tacky, low gloss) or over-cure (brittle, poor intercoat adhesion).
General principles of 2K polyurethane crosslinking and automotive refinish application (public literature).
Kexin New Materials (Guangdong) Co., Ltd. · 2026-09-17