Improving the Appearance of a 3-Coat-1-Bake Coating Film by Minimizing the Shrinkage Difference between the Three Layers

The 3-coat-1-bake coating system has been widely employed in automobile body painting. This study examined whether the appearance (waviness) of film can be improved by reducing difference shrinkage percentage (by weight) between primer surfacer and basecoat clearcoat after flow/leveling stops while baking. To delay time (tC) to reduce clearcoat, solventborne clearcoats were prepared blocking all or part isocyanate group with 3,5-dimethylpyrazole (DMP). tC was measured using electric-field tweezers baking at 140 °C. respective percentages surfacer, basecoat, (ωS, ωB, ωC, respectively) tC. increased as DMP content increased. ωC value is lower than ωS ωB values when zero; however, increased, decreased Wavescan Wa, Wb, Wc, Wd (i.e., appearance) |ωS − ωB| + |ωB ωC| decreased. We confirmed that an effective way improve appearance.