It used to be that heavy cleaners, bleaches, grout erasers, etc. used to get the mildew stains out of our shower grout, but now it doesn't do more than lighten it to a grey color. Is our only option to remove the old grout (which is in otherwise good condition) and re-grout? If so, I assume that we need to get all of the mildewed grout out or else the old grout will stain the new grout from the inside. Is this correct? Thanks in advance for the advice!