Yes, it is recommended to ensure that the clapboards are sealed on all six sides (back, front, top, bottom, and cut ends) to keep moisture out of the wood. Protect the wood with a high-quality exterior-grade paint, semi-transparent stain, or UV-blocking sealant on all sides if possible before or immediately after installation. Sealing the back side may seem like extra work or cost, but in the long run, your siding will hold paint much better and far longer, and save labor and money.
Pre-priming and or pre-finishing is an ideal way to guarantee that your boards are sealed from moisture before they are installed.
