End-grain sealer for the cut edges. Your wood will start silvering down after 6 months to a year whether it’s tanalised or not. You can prevent this with a uv protect oil from Osmo ( yearly ). If your timber stays out of sustained contact with moisture it won’t rot for a very long time so you could, for instance, use un- tanalised shiplap and then oil or paint. DON’T, I repeat, DON’T buy kiln-dried shiplap and go home and fit it immediately. Let it acclimatise, leave it in the open part under the roof before fitting. If you have a dry warm week you’ll need to fit loosely. Ie. Fit the first one, rest the next on top and lift off slightly (1-2mm) and fix. This will allow for expansion due to moisture.