According to ISO 14001 a documented information can even be a graphic. In this case the instructions on wall is internally communicating three key elements
1) Environmental aspect: Accidental spillage of oil based paint, thinner, white spirit, and solvents cleaners [Substances explicitly mentioned]
2) Environmental Impact: marine pollution
3) Control: Human precaution
this is much more than what a graphic can communicate. A change in operating procedure should be a trigger for updating this memory aid / no loose ends / convenience should not be achieved at the expense of risk.
If a single memory aid has a potential to result in EMS failure or prosecution then its better to either control it or get rid of it.
Minor NC / Case closed /