I think this video will help you: https://www.youtube.com/watch?v=QKyV9jzh0LM" rel="nofollow noopener" target="_blank"> https://www.youtube.com/watch?v=QKyV9jzh0LM
Hi Pamela, this should help
Dust the wood with a dry paint brush. Make sure to clean out crevices. If there is a lot of mildew and it is dry, you can also use a mildly abrasive sponge or sandpaper sand off the mildew stain.
Put on safety glasses and rubber gloves. Ensure that your area has adequate ventilation before you proceed to the next step.
Mix household (laundry) bleach with water in a clean bucket--1 cup to 1 gallon of water. Water can be cool or warm. Hot water more easily warps wood--avoid using hot water.
Wet a sponge in the bleach solution. Rub over the mildew stain, saturating it with the solution. Allow this to remain undisturbed on the wood
Dry the wood with a clean towel. Turn on a fan and aim it toward the wood. This will promote further drying (to prevent warping of the wood).
Pamela, Try Murphy's Oil Soap, it's just for wood. I'd wipe it down first with a rag, and get as much off as you can. This product is sold in supermarkets, in the isle with cleaners. After that I would use Wilbert's lemon oil to protect the wood. Follow the instructions on the back. This too, can be found near the oil soap. Aloha!
You need to use vinegar. Bleach will "bleach" the staining, but not kill the spores. Vinegar kills mold spores, bleach does not.
Once clean and bone dry, it would be recommended to seal them to prevent future damage. My first question was why were they left untreated? Open invite to pests and termites.
Wash it with the bleach water, getting into all the crevices using a toothbrush if you have too to clean it really well. Let it dry, then spritz on vinegar to kill the mold/mildew spores that may not have wiped off with the bleach water. Like Naomie states, bleach does not kill the spores, it will remove loose spores and help with the staining, it will not kill the spores. Another of the million uses for vinegar, great antibacterial and antifungal agent. Once clean and completely dry, seal the wood so this is less apt to happen again.