This Spring I am planning to repaint the extirior of our home. It has Cedar Siding that is painted Biege...I plan to repaint it Gray. I know I have to scrape and Prime any area that is peeling or has exposed wood. Do I need to Prime the entire house before painting ???....most of the Beige paint is in good shape, but the weather side of the house is peeling.

Its always best to prime all walls before painting. Power washing a few days before the sanding process will remove any mildew and dust that has collected on the surface of the cedar siding. Once dry prime using a good quality primer. There should be two coats of primer. One to seal the bare wood and bring the paint level up to the current paint surface. Then when that dries a complete coat of primer on the entire wall surface before you put the final coat on. If you do this you will hide the sanded spots better then just using one coat.

It is always suggested that you obtain the advice of the paint manufacture on how to properly prep the walls before you paint. I would go to a professional paint supply store for that advice and not the big box stores as they may not have the experience needed to properly guide you.