On 26/04/2012 17:44, Andrew Gabriel wrote:

Depending on the quality of the back of the mirror, the acetic
acid given off during curing of silicone can wreck the silvering.
There is a special mirror equivalent which does not risk damaging
the mirror silver. A glass shop will have it, but I think I've
also seen it in one of the sheds.

There are actually different sorts of mirror adhesive, as I found out
not long ago. Both come in cartridges; one sort is similar to silicone,
and you apply thick parallel beads of the stuff all down the back of the
miror; when you apply it to the wall it needs the weight supporting for
a good while till it cures (I use a batten screwed to the wall) and you
need to take care that there's a decent thickness of adhesive twixt
mirror and wall (I use spacers behind the mirror to ensure this), and
that there's free flow of air to all the adhesive so it all goes off OK
(ie a big spiral or concentric circular beads are not good).

The othet type is an impact adhesive; you spread a thin film on both
wall and mirror, wait 5-10 mins and apply the mirror to the wall; and
that's it, stuck. No support needed.

I prefer the former type myself as my walls are rarely (never)
billiard-table-flat, so I'm more confident that the adhesive is actually
doing its job!

David