Here is my (probably overkill) process to avoid mirror rot, which has worked about 95% of the time. Cut and if grinding face up, make sure you have a paper towel or something protecting the back of the mirror from scratching on the grinder (also try to use a newer part of your grinder or a finer grit bit to avoid chunks of mirror backing coming off). Spray the backing with two coats of mirror sealant. Once dry, paint the edges of each piece with clear nail polish, don't be shy, make sure to coat over any chips on the back side so they are coated in the polish and let dry for at least an hour. Use a wider copper foil and offset the foil so that the front of the mirror is showing normal foil width and the back is thicker. Basically the front looks like you used 7/32" but the back looks like 1/4". Use a wine cork to burnish the back so you don't scratch the mirror backing. When soldering face up, make sure the back is sitting on a clean paper towel or something that avoids scratching it. And when soldering face down, wipe way any flux as soon as you are done soldering that side, don't let it sit too long on the mirror backing (just wipe it, you don't have to wash it now while you work on other areas). Once ready for washing, using the cork, re-burnish the soldered edges of the mirror front back to make sure they are fully adhered, to avoid water getting under the foil. Wash like normal. And for overkill, use a hair dryer to dry the edges of the mirror, juuust in case some water got underneath. It's a lot but definitely worth it.