If you have Masonite or hardboard siding, it is important to keep it sealed with paint. The siding is primed when it comes from the factory, but problems can develop after installation. Each nail must be caulked before painting. Special care must be taken to get a good coat of paint on the bottom edge of the laps and the full sheets.
Most of the water damage to hardboard siding comes from raindrops that "hang" on the bottom edge. If the edge is not sealed with paint, the water will wick up into the siding.