2. Clamp the two ends of the stripped film（PU paint） with the index finger and middle finger, press the stripping surface of the film with the thumb, and gently rub the two stripping surfaces of the film on the clean wall;
3. Lay the film flat on the table（PU paint). If the curl of the film is strong, press the two ends of the film with a heavy object;
4. Dip enough dye with a cotton swab and apply it on the rubbed stripping surface and its periphery.
5. After 10 ~ 20 seconds, wipe the applied dye with toilet paper.(PU paint)
6. The side with polyurethane adhesive layer on the film will show blue traces, and the color development area is consistent with the rubbed area. The depth of the blue trace is related to the thickness of the adhesive layer.
