Let's look at what it takes to properly prepare an exterior surface.
First, the surface to be painted needs to be clean. Power wash the house or scrub it down using a stiff brush or sponge with a mild detergent and warm water. Be sure to remove any dirt, rust, grease or any other contaminates that could prevent the paint from adhering to the surface. Clean in small sections and rinse before the detergent and water can dry.
If the existing coat of paint still appears chalky, you should wash the area again to remove all of the chalk. If you don't, the new coat of paint won't adhere. You wouldn't wax your car before you wash it, so don't paint your house before you wash it either.
Next, any loose or peeling paint must be scraped off, followed by sanding to smooth the surface. After sanding, remember to wash off the sanding dust. Areas around joints, doors and windows are susceptible to peeling, so pay particular attention to these areas.
The next step is to spot prime any exposed wood. The primer penetrates the surface and acts as a sealer. The paint then lies on top and protects. Also at this point, make sure to caulk or patch any cracks before you begin to paint.
Before applying a new coat of paint, make sure the surface is dry. If it's not, water vapor will try to escape through the paint film and blistering may occur.
And finally, the surface to be painted must be dull. The surface should be roughened up to give the new paint good adhesion. This can be done by lightly rubbing sandpaper over the surface. Again, be sure to get rid of the sanding dust before painting.
There you have it... the basic steps to proper exterior surface preparation.