If your addition calls for clapboard siding, for instance, you can save more in the long run by ponying up now for the preprimed and prepainted variety. It costs an extra 10 to 20 cents per foot, but "you'll wind up paying for half as many paint jobs down the road," says Paul Eldrenkamp, owner of Byggmeister, a design-build remodeling firm in Newton, Massachusetts. The reason? Factory finishes are applied on dry wood under controlled conditions—no rain, no harsh sun. "I used prefinished claps on my house about ten years ago and the only flaw in the finish is the occasional mildew spot, easily washed off," Eldrenkamp says. "The paint looks as if it'll be good for another ten years, easily."
Lucky for you, there are paints to help. Rust-Oleum makes a "mold and mildew-proof" paint for about $27 a gallon that can protect those mold danger zones. You can fight those nasty germs with Sherwin-Williams' Paint Shield, which costs about $85 per gallon. Unlike other microbial paints, this formulation is EPA-certified to kill more than 99.9% of surface bacteria in less than two hours, including E. coli and staph.