Your house now may have many cracks in the concrete that you may not know about for years. You have to be persistent. The nails that attach the siding have grown rusty, and the rust bleeds through the paint even when I use a good stain kill. To get rid of the rust spots, counter sink the nail heads and fill the space with an exterior spackling compound. I think that Sears makes great hand tools, but disregard the company's advertising jive.
Proper prep is the most important step.
Everyone wants to paint in the spring, but wood needs sufficient time to dry out after all the spring rains. Before you paint, go to a full-service paint store and rent or borrow a moisture meter. Check the amount of moisture in the wood. The magic number is 14. To me, this means that the product may not be environmentally friendly and that one is washing poison into the ground and the local water supplies.