I'm not a fan of graffiti, but if I were an artist who wanted my art to last, I would seal the finished piece with a hydrophobic spray. Liquids do not stick to it, and it could not be painted over without taking a belt sander to it first.
Hello. I'm a creative sort of a fellow, and I'm always coming up with ideas. I have notebooks filled with them. I'll be posting them here from time to time. If you like one and wish to adopt it, feel free.