Mould on wood after staining

[Susan Campbell]

I have sanded my interior window sill back to the bear wood to take off old varnish and sand out scratches. When i put on first coat of Ronseal varnish lots of little black mould spots appeared all over the wood. I sanded it back again taking out all the black and started again but the same happened. I have sanded and revarnished many window sills and never had this happen before. And no its not the steel wool!! Can anyone tell why this is and how I can solve it?