Blue fungus staining your external timbers? It doesn't have to be this way...

Blue stain on external woodwork is an all-too familiar sight. The problem is so common that many UK woodworkers assume it to be unavoidable, at least when working with softwoods and hardwoods. In this post, we'll show how you can minimise blue staining...or avoid it completely.



Blue stain, or sapstain, is a microscopic fungus occurring naturally in most heartwoods. In the growing tree, it has no opportunity to develop. When the tree is cut down, the fungus emerges on any exposed surfaces of the lumber where it can obtain an air supply. Its growth, generally rather shallow, will be controlled by drying processes, and virtually eliminated when the wood is sanded and planed.


While fungal spores are likely to remain dormant in the wood, the application of a varnish or seal to the finished surface should in theory block the ingress of air and thus prevent future germination.


However, as anyone who has examined hastily-assembled timberframe buildings will attest, things don't always work out that way. Blue sta