Just finished reading Vespa forum posts about all the seam and underbody rust they have to deal with - especially in the U.K.
Seems to creep in no matter how they prep and/or clean them.
I'd of thought that, for the price, Vespa would be spraying on a very, very strong and thick substance to coat the bottom of these scoots.
Anyway, the most rust I've seen on a scooter was all the discolored bits (rust?) on nearly every weld on the grey painted frame of a NOS Burgman 400.
Kymco , Honda and Piaggio paint their frames gloss black - so I'm not seeing any rust there. I've treated all the metal bits and areas I can get to with ACF-50 on each of my scooters. And of course our plastic panels will never rust!
The ACF-50 smells funny until it cooks in - but seems to be protecting well.
Stig