If you live in the area with a lot of humidity, you will notice that your outdoor wood is split, discolored and ugly. Luckily, there is a solution for this problem. Humidity is the main cause. But there are many man-made solutions that can help you. Many people use copper chemical compounds to protect their outdoor wood. This will preserve it through almost all weather conditions. In those cases the wood is treated with chromated copper arsenate (CCA). If you are buying a lumber, make sure it is pre-treated with copper compounds so it can resist to high humidity conditions. If you want to apply it yourself, it requires a lot of time. Basically, you will have to paint outdoor wood with copper solution. CCA provides one more benefit.
It is good in preventing UV damage. This will protect the wood from fading. Treated wood will have that greenish look, but it can be covered with stain.
Another effective way is water repellent. Because humidity is moisture, they are very efficient. Usually they are oil-based or wax. These materials repel water so it rolls off the wood rather than sinking into the grain. You should know that water penetrates wood, it will cause splitting, swelling and further damage. The best water repellents are those with fungicide. They will also prevent the mold growth and bacteria.
After you have protected your outdoor wood, you will have to apply finish. There are several options for this. Paint is the most common and according to many people, the best finish. Others ways include clear sealer or strain. You can do the finish yourself, there is no reason for hiring a professional.