This is due to adhesive being left behind on the metal sheet.  Clean the metal sheet as per the below instruction:

1. First, gently wipe the adhesive off by using an alcohol swab or cloth containg rubbing alcohol. 
   For best results, use wipe in the direction as per below image.
   
   　
   
    
2. Wipe the adhesive off by using a cleaning cloth (i.e. rag or paper towel) while the metal sheet is still wet. 
   For best results, this must be done quickly while the metal sheet is still wet.
    

   Repeat step 1 and step 2 until the metal sheet is clean.