How to remove brown stain from retainer?

Pour the vinegar in a small non-metal bowl and let your appliance soak in it for a few hours. The acid is strong enough to help break up the proteins, odors and other debris that may accumulate on your guard. A word of caution, though: if your appliance also has metal in it the vinegar can cause it to corrode slightly, so you probably shouldn’t use this process. This is when you’ll want to use a denture product specifically labeled for use with partials (typically dentures with metal parts).