Gently dust with a soft, dry brush. If grime persists:

  1. Mix a few drops of pH-neutral, olive-oil soap in distilled water.
  2. Patch-test a corner.
  3. Lightly wipe, then blot with a damp cloth.
  4. Air-dry—never heat dry.
  5. Skip abrasives and harsh chemicals; they scar the painting / varnish.