It is strongly advised not to stain over mold on your newly sanded Douglas fir deck railing. Mold growth is a sign of excess moisture and can lead to a variety of issues if not properly addressed. Staining over mold can trap the mold beneath the stain, allowing it to continue to grow and potentially damage the wood over time.

To effectively address the mold issue, it is essential to first eradicate the mold from the wood surface. This can be achieved by thoroughly cleaning the affected areas with a mild detergent and water solution, followed by a mold remover or a mixture of water and bleach. Ensure to wear appropriate protective gear such as gloves and a mask when dealing with mold.

After cleaning the mold, allow the wood to fully dry before proceeding with staining. Utilizing a moisture meter can help determine when the moisture content has dropped below 15%, indicating that the wood is ready for staining. Additionally, consider applying a mold inhibitor to prevent future mold growth on your deck railing.

Taking these precautionary measures will not only ensure a successful staining process but also contribute to the longevity and aesthetics of your deck railing. Remember, proper maintenance and care are crucial to preserving the beauty and integrity of your outdoor wood structures.