Do I need to remove mould from plywood before treating?
Yes, you should remove the mold. It seeps into the wood and would prevent your sealers from doing their job to the fullest. Bleach is good for surface mold but it doesn't penetrate the surface. I've used white vinegar, one cup to 3 quarts water. Scrub the mixture on the wood and then spray off with water. Let the wood totally dry and follow the directions for your preservative. Good luck and stay safe out there.