When did bette nesmith graham invent liquid paper?

She remembered that artists painted over their mistakes on canvas, so why couldn't typists paint over their mistakes?

Invention of Liquid PaperBette Nesmith Graham put some tempera waterbased paint, colored to match the stationery she used, in a bottle and took her watercolor brush to the office. She used this to correct her typing mistakes.

from Mary Bellis at About.com