It's still amazing to me how many homeowners I run into that don't understand what chalky paint means? Go out and rub your hand against the exterior walls of your house and if your hand looks like they are covered in chalk or powder it's time to repaint your house. It's not ok to paint over chalky paint with new exterior paint. Chalky paint or chalky powder is a result of weathering and breakdown of the paint. Paints will become chalky at some point, depending on the age of your paint and exposure to the elements, such as sunshine and water.(Welcome to Florida) If chalky paint is not removed or sealed with a proper sealant, it can eventually cause damage such as moisture permeating your substrate which cause masonry cracks in your walls. If you just paint over chalky paint these things will most likely occur; paint discoloration, paint peeling and void of any manufacture warranty your paint has. It is absolutely necessary to remove all chalky paint, if this can not be achieved by cleaning the surface than a special sealant must be applied to seal up the weathered paint before applying any new paint. This is the biggest investment you own and yet most homeowners still spend the least amount of money, research or care on it?  That's why you should always hire a professional licensed painting company to paint your house.