you can try any type of brush you like, it can be a long hard slog esp. If it has lichen on it as well, stiffer brushes don't really fair any better than medium or soft - you need a good solution of g101/ubik/TFR to work into the grain. Having now done lots of fibreglass caravan roof cleans I pressure wash them using a mini flat surface cleaner, get great results and takes about 20mins to do
Darran