I have found that the "problem" with a marker is that it will first turn purple and then fade. It will also not cover silver (or other) leaf completely and it will fade in those areas much more quickly leaving a blotchy reflection.
Jim- if the mirror is beveled, then there is only a very small contact area immediately adjacent to the lip. How do you apply the foam tape evenly (without stretching) or getting it wavy and also have it close to the lip? What happens to the foam/adhesive if the mirror is in a bathroom and water splashes up against the mirror and runs down? If the mirror is "sealed" against the foam, will the water swell the finish on the mirror if it is not immediately wiped off?
Questions:
Do any of you seal the edges of a mirror before it is fit (or order it sealed from your fabricator?) I don't mean put a backing on the frame, I mean treat the edges of the silvering with something like Gunther Mirror Edge Seal to prevent black edge from forming (especially in a bathroom?)
Do any of you use "setting blocks" on the bottom edge of the mirror to hold it off the inside edge at the bottom so if water does get in, the edge does not sit in the water and cause the silver to delaminate (black edge)?
Both of these products are available from CRL (and they are a great resource for all things glass and mirror).