Ditto what Dale said about the sides of wood.
What I have had outstanding success with over the years, is Thompson's "Wood Preserver."
This product goes on clear and absorbs into the wood, and disappears. Apply 2 coats, and especially around the edges and in cuts. Once it's dry, you almost can't tell anything has been done, except for a slight darkening of the color.
Not sure what's in it, but it's stinky stuff, and it really works. Better than lacquer, urethane or epoxy, in my experience, as it's not sitting on top of the wood, but is actually inside the wood. So it can't crack or peel off and let water in.
We used to build sheds out of untreated pine T1-11 wood. Once all the cuts were made, we would use a garden sprayer to soak the siding with wood preserver. Those sheds lasted many years with no rot. Most people thought the natural wood was so pretty they didn't even paint them. We would recommend re-treating them every 2-3 years, as rain would splash mud on them, and leaves would build up around them and so on. But in all those years, never had a customer tell us the shed was rotting.
Since then, have used that product for anything, and it is simply outstanding. In boats, I have used it for the decking, and so far, none have gotten soft. The more years I use it, the more confident I am in it's effectiveness.
One cool thing about it that I really like, is that after it dries, you can use glues, adhesives, contact cement, and so on with no issues. Yet, spray water on it, and it beads off, like a waxed car. GREAT stuff!