Cleaning and sealing any outdoor wood is not much different than doing the same thing for a wood deck. Outdoor wood furniture, playsets, arbors, etc. all take the same abuse from the weather. Specifically water and UV damage.
If you fail to maintain outdoor wood it will eventually rot and crack to pieces. Chemically (includes: Thompson Water Seal, Spar Varnish, ets..) treated wood is able to resist rot for decades. Some wood like redwood, cedar, teak and other hardwoods contain varying degrees of natural wood preservatives. But water that soaks into all these wood species will cause them to crack
The cracks start out as tiny checking cracks. Water entering the wood causes it to expand. As it dries, it contracts. This movement creates the tiny cracks. If water gets into the cracks, it soaks deeper into the wood causing even greater expansion and contraction forces that cause the cracks to get deeper and wider.
You can see why it's important to treat all wood with a water repellent on a regular basis to prevent water from getting into the wood. Do this every other year or so and your outdoor log furniture or playset will look fantastic instead of resembling a 50-year-old fishing pier
If you are planning on using the logs in a museum or play-space, there are some options that will provide more protection over time. We recommend using a Spar Urethane Coating, since this protects logs from both UV rays and water.