Vinyl decking is a popular construction material due to its resilience, its affordability and its relatively low maintenance. Unlike wood, vinyl deck railing is nonporous and weather-proof and can be safely cleaned with almost any mild cleanser. However, because vinyl deck railing is often exposed to the elements, it can sometimes accumulate an unsightly layer of mildew. Cleaning this mildew is very simple. Spray mildew with straight white distilled vinegar. Allow vinegar to sit for 10 to 15 minutes. Wet a sponge with hot or warm water. Sprinkle sponge with baking soda. Scrub vinyl decking with sponge until mildew is loosened. Wipe mildew away with a wet rag or hose it off.