Take said wheel off... get some fine wire wool and harpic toilet cleaner. sit down the floor with a hot cuppa, away from harmful sun rays, wear some gauntlets and start by wetting the wire wool in harpic and hot water scrubbing spoke by spoke. The spokes on these bike are not stainless steel they are just common steel they get dirty, and show corrosion easily.

Best thing to do is to repoke them on same rims with more expensive spokes.