Small random-orbit or pad sanders make this job go faster. (Wallis first covers these boundaries with Synkoloid patching compound so no edge is visible after sanding.) As shown, you want to make sure that there is a feathered, smooth transition from exposed wood to old paint. For areas that might get close scrutiny, you can follow up with a 100- or 120-grit rubdown to erase any scratches.
The last big decision is how to apply the paint. Most pros use paint sprayers because they're fast, but in inexperienced hands a high-powered sprayer can leave drips, thin coats, and a mist that may land on many things other than your siding. If you do hire a painter who uses a sprayer, make sure he is meticulous about removing, covering, or masking off everything in the area that might get hit with overspray: gutters, roofs, windows, shrubbery, walkways, cars—you name it.
A number of factors help determine when you should paint your home’s exterior. These include your local climate and weather conditions, the age of your home, and the type of siding used. A high-quality finish in ideal conditions could last 10 years. But if your home takes a beating from the elements, you may need to repaint as often as every three years. Most homes can be on a 5-8 year interval. If you’re not sure if your house is due for a refresher, watch out for signs of deteriorating paint, such as fading and chipping.