Originally Posted By: demarpaint
I have an 11 hp power washer, and have seen first hand what kind of damage these machines can do in the wrong hands. That is the only reason why I suggested being very careful. I wouldn't scrub it off either, maybe I should have said that, but I figured the OP already knew that, I hope.

I think repeated soaking with a good hot soapy mix followed by a garden hose blast might be the ticket.
A little OT but maybe worth mentioning, for at least entertainment value. It made me smile! I had a customer call me about doing a cedar deck. Power wash it and re-stain. I quoted a price, and didn't get the job, IIRC I was told my price was double the competition. Typical in this economy. About a month later I get a call back, "can you fix my deck?" Well curiosity got the best of me, and I had to have a look. A new guy low-balling, going to set the world on fire, wrecked her deck. My suggestion to her was get an attorney.
All of your points are important for the OP to take into account if he uses a power washer. You want to leave the paint in place and only remove the poo
I would probably use a 15* or 40* tip on my lowly 5hp power washer (probably not even at full pressure) if I was doing this job.