hi, i recently cleaned my deck of mold stains and mildew and i want to restain it, but some stains refuse to come out and ive tried everything. also the old stain is still in places. do i need to sand down the old stain areas? or can i put on a new coat the same color and it would be ok? do they make any kind of deck restorer that would work better? i attached too pics of the area.