I was wondering how the French make a levain. If I had some sourdough in 100% hydration, can I make a levain out of it? Do I need to add yeast?

I am making a chocolate wedding cake decorated with chocolate vine leaves and fear that even the best-tempered chocolate may develop a bloom in this damp weather. I have heard of a glaze that professionals use to give a shine to chocolate, but I have not been able to find any. Does anyone know of a supplier?