Bleach in a spray bottle - esp for nonskid. Or, if you like to scrub or need to confine the application, soft scrub with bleach. Either way, it's the bleach doing the work.
Another vote for bleach, I also add a little dawn anti bact dish detergent. They sell a nice brush at Stop and Shop, its about 2' long and has green bristles, I thing it's for a toilet bowl, works pissa
Hybrid Mode
