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You shouldn't paint a metal fence. Painting doesn't necessarily damage a corrugated metal fence, but these types of fences are meant to flex, and paint can easily chip and break off. Latex paints can address this issue, but those tend not to be very weather-resistant. Certain paints can trap moisture in the corrugation, which can rust the metal and lead to corrosion. And finally, if you paint a copper fence, it won't patina, which is one of the main reasons people buy copper.  

If you don’t treat a cedar fence, it will turn from reddish brown to gray over time. You can keep the cedar from graying and retain its attractive, deep red color by sealing your cedar fence with a stain that keeps the wood’s oils in and serves as a barrier against the sun’s UV rays.

Wood stain does go bad, but it will last in the can for between three and six years, provided you use proper storage techniques. Storing wood stain in open containers or in extremely low or high temperatures will cause your wood stain to become unusable much more quickly. Water- and latex-based wood stains typically last for just two to four years, while oil-based wood stains can last for three to six years.

If it rains after you paint your fence, you may need to apply another coat of paint after the fence thoroughly dries. This will ensure a good seal if the rain weakened any areas of the paint. However, if your paint had enough time to dry thoroughly before it rained, you should have no problem.