There are many sites that offer advice on this matter. This one makes some suggestions that might help.http://forums.iboats.com/boat-topics-qu ... 62709.html
Here's a forum about pressure washers on fibreglass. MBounds caution seems to be appropriate. But if a wide tip is used sufficient distance from the surface then it could be OK.http://www.thehulltruth.com/archive/t-439439.html
This is a link to some ideas about re-finishing fibreglass.http://www.biokleen.com/diy-boat-cleani ... ation.html
I got all this stuff from Google, I have no personal experience of dealing with a problem of this severity.