Scraping and power washing is a major part of exterior prep. Which comes first depends on the condition of your home.
I like to sand the scraped areas, this produces a better look. Sanding the rough paint edges will produce dust. If your home requires extensive scraping and sanding, you can wash after the sanding. This will remove the sanding dust as well as the dirt and gunk.
After the scraping and sanding is finished you need to prime all the bare areas. You can also prime the entire house is the old paint has developed surface cracks.
Another reason to prime is if the original paint is an oil base and you want to apply a modern acrylic. In this situation, it is best to prime the entire house. This way you know the new paint will stick.
1) For general spot priming I use a standard oil based wood primer.
2) For cracked paint I use a bonding primer. This type of primer penetrates into the crack and glues the edges of the paint. Two good penetrating primers are Peel Stop and XIM Peel Bond.
3) If priming your entire house, you can use either the oil base wood primer used for spot priming or a very good acrylic. The XIM Peel Bond can also be used for whole house priming.
After the primer has dried, you can caulk all the gaps around the windows and doors. Plus open seals on the soffits, fascia boards and trim or any other area needing a good seal.
Now you can paint. Check out <a href="https://www.house-painting-info.com/exterior-paint-preparation.html">Exterior Paint Preparation</a> for more details.