Wooden cages are fine and are used to build ferret cubs or hutches for ferrets housed outdoors in the UK. It's not necessary to paint the wood, instead use timber treatment sold for use on garden fences and outbuildings, and it has to be 100 % non toxic, with the manufacture's instructions followed implicitly. Never use creosote, it is poisonous. Ferrets need hard flooring, not mesh.
Refer to the related question below "How to make a ferret cage"