cuprinol clear, to protect against woodworm and rot. Take them off so you can treat the underside and other concealed parts, which will be neglected in future. The timber within a foot of the ground will suffer most from rainsplash so needs multiple extra treatments.
Once fully dry, a water-repellent coating. Decking oil would do, or a light-coloured shed and fence stain (one that is suitable for planed timber). You will need to retreat every few years because it weathers off.
I'd also suggest Roxil Wood Protection Cream. Easy to apply and nice results. Check youtube on how it looks while applying and how it stays afterwards...
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