I haven’t had a problem with mold myself, but I have a good idea as to what caused this problem in your last batch. Plastic pails can scratch, and it can make the bucket harder to sanitize. Be especially careful when cleaning the bucket; never use any kind of abrasive brush or pad. Try not to store anything metal or hard in the bucket like cans of liquid malt extract or metal stirring spoons as they could gouge the plastic.

A scratch might not seem like a big deal, but it may prevent the sanitizer from reaching the exposed plastic. A primary fermenter is an ideal environment for mold – warm and wet. Examine the bucket for scratches before sanitizing. Also if you’re using bleach to sanitize I would recommend switching to a no-rinse sanitizer like Star-San. There is no rinse step, so it reduces the possible contamination points before sealing your bucket.

Side note – Make sure your bucket is made from PET food grade plastic. I’m not really sure if buckets from Home Depot are, but it’s important to check (not because of mold, but non food grade plastic can essentially leach into your brew and ruin it).

I’ll do a post on this for you later today, and I’ll email you the link when I finish. Good luck on your next batch, let me know if it continues to be a problem.