The mannen-gami (eternal paper) is a memo paper coated with a transparent Japanese lacquer and written on with a writing brush, and as it's name indicates from its characters, a letter written in sumi comes off when it is wiped with a damp cloth and it can be used for many years.
万年紙は、透明な漆を塗布した紙で、墨筆で書くメモ用の紙で、湿った布で拭けば墨字が消え、長年に使えるので万年紙の名ががある。