The inner belt weatherstrip typically generates noise on rainy days; it is closely related to humidity, particularly when the glass is wet (Ma et al., 2021). Under wet condition, the friction coefficient is lower than that in the dry condition. However, when the glass is repeatedly lowered and raised, the water dries, and the friction coefficient converges to that of the dry state. However, a separate phenomenon exists, where squeak occurs owing to a rapid increase in the friction coefficient after a few cycles of operation or sufficient drying time has elapsed. This state is called the semi-wet state. (Oumohand and Sartoni, 2012). This is presumed to occur when the difference between the friction coefficients of wet and dry states is large. The wet area tends to slide easily, and the dry area tends to slide less owing to the relatively high friction coefficient.  Consequently, the flocking surface needs to be modified to reduce the difference in the friction coefficient between the wet and dry conditions. 
It is difficult to simulate wet conditions using Ziegler’s SSP-04 equipment because the friction surface is perpendicular to the direction of gravity. Therefore, the equipment and test method suggested by Oumohand and Sartoni (2012) were used, except that the normal load for the test was 2 N, and the test speed was 50 mm/s. Consequently, the difference between the wet and dry friction coefficients could be reduced by increasing the coating amount from 2 g/m2 to 5 g/m2. This can be attributed to the larger drop in the dry friction coefficient compared to the wet friction coefficient, as shown in Figure 10. Because both manufacturers used amino-modified silicone to control surface friction, the performance difference between coatings from different manufacturers was insignificant