Some furniture (particularly furniture living on the coast and exposed to heavy mists) can form a residue over time. Iron oxide stains in wood are usually dark spots caused by prolonged exposure to water or metal. The iron oxide reacts with the tannin in the wood and stains it black. The best way to restore this would be to use Oxalic Acid. You can buy it from any hardware store – then simply just dissolve it in water and brush the affected area. Place the chair in the sun to dry and re-varnish it.