How do I prevent mold in drywall mud pail?
Once you have mold in the bucket, the compound is no good.  After using some compound from a bucket, I found that if I cover the unused compound with cellophane it helps to extend the life of the bucket.  Take a bucket scoop to push all the unused compound into the bottom of the bucket, then cover it with the plastic.  Even though the compound may last a little longer, it won't last forever.