When applying clapboards as exterior siding to your home, it's a good idea to prime both sides of the clapboards before installation. By treating the backside of the clapboards, the cut ends, and the exposed face, you're adding a moisture barrier to all surfaces of the wood. Water damage is the chief cause of premature deterioration of your house exterior.
For more on siding, consider:
Wood Siding Options
Adding Clapboard Siding (VIDEO)
Quick Tip: Installing Clapboard Siding