All finishes with organic elements like linseed oil will develope mildew eventually. The newer generation of polyuerethanes probably a better choice for the "varnished" look. I prefer a product called Sikkens for exterior wood treatments of any kind due to it's U.V. defeating properties. It's expensive but it's longevity has no equal. Some parts of my house exterior, all of whci is western red cedar, look as good as when it was applied 10 years ago.