Used Milton on my hood, just make a strong mix in some hot water say 3 or 4 capfuls should be strong enough to treat any green growth/staining. Use a nylon nail brush to agitate it into the grains of the roof material following the grain. Rinse with clean water and allow to dry. I use Fabsil tent waterproofer and use a paint brush to apply it to the hood. You need to mask the windscreen and side glass to prevent overspill. Pays to lay something over the bonnet as well as the boot to avoid splashes landing on the paintwork. Some have used Fabsil Gold which by all accounts is better than the standard Fabsil, I will try that next time. Once dry you will notice water then beads on the material. Both my Z3 and Z4 have the Fabsil treatment.