This is the same insulator after I left it to soak overnight in sulphuric acid. This is my favoured method of cleaning stubborn marks. The following morning I rinse the insulator in water to remove the acid and I wipe it with a wet cloth and any paint/soot/cement comes off instantly. You would never think this insulator is the same piece as in the previous picture.

One word of warning - this acid is strong and I have accidently got my clothes soaked in the past and it eats the fabric. When the clothes dry out they become brittle and fall apart when you touch them. I believe the same thing happens to human flesh but I've never tried it yet.