Weatherproofing your lumber retaining wall is extremely important to keep it serving its function for a long time. Because the retaining wall is used to keep soil or rocks from moving from a higher area to another lower area, it is key that the wall be stable and in its best shape. In order to do this, there are a few steps that you need to take.
Step 1 - Wash the Wall
Begin by washing the wall completely. You may just need to use a hose or power wash it to get it clean. This way, you can start out with a fresh, clean wall to keep it from molding or deteriorating before you stain it.
Step 2 - Stain and Seal
Once the deck has completely dried, you will need to stain it. In order to do this, go to your local hardware store and look for a wood stain that is made for outdoors. This process is done for a couple of reasons. First of all, stain makes the wall look finished, but the real reason for this is that the stain helps to protect the wood from bad weather. Once you find a stain that works for you, you might also want to ask about a sealant. The sealant is just another protective layer that is between your lumber retaining wall and the bad weather.
When you have your materials, you will want to pick a day to stain and seal that isn't raining or snowing. Try to find a block of 2 to 3 days, depending on the size of the wall and how long you think it will take you to stain. Begin with the stain and use a paint brush or a roller to brush it on. Allow it to dry for a couple of hours and then apply another coat. Typically 2 coats is good for a stain. Then, if you plan on sealing the lumber retaining wall as well, you can apply a coat of that. Once you are all finished, you will need to allow the retaining wall to dry for a few days. This is why it is important to pick a few days that aren't supposed to have rain.
Step 3 - Note the Day and Maintain
While it may seem silly, you will want to make a note of the day that you stained and sealed your wall. Typically you will need to stain again in a few years, depending on what kind of weather and climate you live in. Five years or so is a pretty good time line to follow when it comes to staining and re-staining, however, you may want to make it closer to 3 or 4 if you live in an area that gets harsh winters. Cleaning the wall on a regular basis with a spray down helps to maintain it as well and keeps it in shape so that it can stand up to harsh weather.