What is a good way to weatherproof a bike outside
Just leave it alone. 

If you have a steel bike and theres gaps in the paint, the rust is likely cosmetic, but you can carefully sand and prime and paint the area. Aluminum and Carbon fiber shouldn't have this problem. 

Oil your chain regularly, and replace parts as needed -- your drivetrain will rust out quicker but that's the nature of it. Rain, snow  and salt tend to degrade your drivetrain quicker than nice conditions, but theres nothing to do about it unless you bring the bike indoors.