A composition for coating of a surface intended to be exposed to a metal melt consists of 8-18 wt-% of a refractory component, 50-75 wt-% of solvent, preferably water, 10-20 wt-% of an inorganic binder, 0-10 wt-%, preferably 2-10 wt-%, of an organic binder, 0.3-7 wt-%, preferably 2-6 wt-%, more preferably, 3-5 wt-% of pyrite; and optionally up to 10 wt-%, preferably up to 5 wt-%, of additional additive or additives. The composition results in a coating on a surface, which coating is able to reduce the dissolved elemental magnesium content in a metal melt to which the surface is exposed.