I have a number of windows frames made from a tropical hardwood that looks like oak. What should they be treated with to protect them? Should I use a modern varnish or paint or are there any more natural hardwaring products available? (I would rather avoid paint and see the natural wood) I want something that will last as once the scaffolding is down we will have to abseil to access many of the windows. The property is 250 year old 5 story tower but the window frames were new about 15 years ago, all existing treatments have been removed.