I would return a new AS to the dealer for a warranty repair. If it is something you want to repair on yor own, yes you need to seal the back of the rail. Trempro 635 is what I like, and is used by several here.
Available from several sources, like Vintage Trailer Supply, at this link:
May also be found at some local fasterner and high end hardware stores.
A "medium" reach rivet should be fine.