How can you weatherproof artificial flowers?

Not very well for outdoors. Your best bet is to take care with location since UV is what does the most damage and keep out of direct Sun. How the color is retained is another issue that dictates fade time. Plastic ones are colored on the surface but fabric absorbs it so colors last longer but should be scotchguarded unless labeled for outside so you can hose off the dust once in a while. If you want low maintenance beauty, you can always try something like cactuses that take little care.