Hi Rob
Bleach is your friend as most of the problem is mould - If very dirty I'll use a strong mix (about 6%), if not I'll start at about 3%. Your s/cleaner needs to be between 1000 & max of 1500psi - don't be afraid of the overlap. Then rinse a lot, sometimes twice to get rid of all the dirt & paint (I use a 2540 at 7gpm)
https://youtu.be/Dn73yq9V0-c sorry about the link, I can't remember how to embed - i really need to get back here more often