Preparation and sanding:
Once you have decided to paint your bike you'll have to take it entirely apart to get a really good job. As you remove parts take pictures so 6 months later you'll remember how it all goes back together. Also try to bag parts and label them for future use.
Unless your bike is in very good condition it is a good idea to have the bike stripped to bare metal. This can be done by media blasting, dipping the frame and other body parts, or at home with a paint stripper. The home paint stripping option is only for people with more time than money as Piaggio really knew how to get their paint to stick and getting it off can be very tough.
When the frame is clean it needs to be sanded to a fine finish with emery paper. Wet sanding always seems to give a better finish and the emery paper lasts much longer than traditional sanding. As a rule if you can feel a blemish or dent you'll be able to see it after the paint job is finished.
The final step before laying down the primer is to make sure all the rust, oil from your fingers, and any other grease or oil is removed from the bare metal. I use a product that actually slightly pits the metal surface to allow a good bond with the primer, see below:
Lay down the primer as quickly as you can after using this product as you have to wash the bare metal with water to remove it which invites rust.