Option 1: Use a damp rag with clean water
Option 2: Rubio Monocoat Soap / Surface Care Spray (diluted soap)
Tannin or Stains: "Black spots or tannin stains can occur after wood comes in contact with a strong alkaline solution or moisture and metal. E.g. cans, flowerpots, etc... These black or tannin spots are caused from a chemical reaction to acid and water."
Tannin Remover is a ready-to-use product for removing stains and spots on wood caused by a reaction to acid and water. It can be used on untreated, oiled, and waxed surfaces. It can be also be used on water damaged parquet floors, as well as on outdoor wood surfaces.