We recommend that you use a penetrating oil sealer product. There are several brands available at your local hardware store: Ready Decks® also sells a quality stain sealer called Wood Defender™. It comes in 2.5 Gallons or a 5 gallon container.
When water soaks into wood, it swells and then shrinks as it dries from the outside in. This is what causes the cracks you may see. In general, the larger the board, the larger the crack you can expect. The more swelling and shrinking, the more cracking you expect to get. The oil in a stain/sealer product soaks in to the wood and prevents water from entering. The stain adds aesthetic value and in our Wood Defender™ products also acts as an UV inhibitor.
Applying most penetrating oil stain, sealer like our Wood Defender™ is very easy. It can be brushed or sprayed on with a general purpose garden sprayer. Keep applying until the wood no longer soaks up the sealer.