Why would you add iron filings in the first place? Well, if you used fine steel wool to burnish the surface after sanding, fine iron residues are trapped in the grain. Add water, a chemical reaction takes place and you end up with a black stain. This sample above was left untouched on the left side, but all that was done on the right hand side was to burnish the surface with 0000 wire wool and then dampen it with tap water. The difference is dramatic.
A concentrated solution of oxalic acid will remove the stain very quickly. Oxalic acid crystals are available from finishing suppliers, such as Mylands. Dissolve in water and wipe the solution over the stain. Rinse off the finished surface with water to remove any acid residue. Be careful, oxalic acid is poisonous. Use rubber gloves and eye protection, and don't make up more solution than needed.
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