@MikeMcKenzy: If you knew anything about DaVinci's painting techniques you would know that the way he applies paints makes his brushstrokes so that they are NOT APPARENT (i.e. you cannot see them). He would never paint what one would consider a "line" because it does not exist in nature.

@wint3rhart001: By painting over it, and using equipment that can detect things that the naked eye cannot.