How to prep fascia board for painting?
If your power washer is anything like mine, I wouldn't use it for fascia washing - you'll definitely be able to take the paint off, but you do run the risk of damaging roof tiles or shingles as well as forcing water into places it will not normally get to.

While it may seem a lot of effort, a ladder and a set of scrapers and sanding blocks will be your best bet. This will also leave them ready to paint straight away, rather than waterlogged.