You have to scrape all the loose stuff off. I'd avoid pressure washing
because a) it won't get all the peelings and dirt, b) it may chew up
the surface, and c) the wood will be soaked and you'll have to wait
days for it to dry out.
If it needs cleaned I'd do it by hand. I'd look also for any loose
boards and secure those. Then just scrape the loose stuff, use a
primer for any bare surfaces, caulk as needed, and paint a coat of
finish. Or 2 finish coats if needed.
That fiberboard is famous for peeling paint. If you want, sand the bare
spots with some light sandpaper [120 or 150] before you prime.
Also, you have to expect more peeling in years to come. Maybe it
doesn't happen, but you have to expect that whatever made the paint
peel this time will work on other areas of the old paint in time.