Turns out WD40 is an effective solvent for the glue - both dried-out and
still-sticky - on an unidentified flavour of double-sided foamy tape.

How did I discover this, I hear you ask? Well... the place we're coming
up to 5 years in has some built-in wardrobes, with mirrored doors
(smoked, unfortunately, which reduces their utility). I guess they were
put in maybe 10 years ago - they postdate the house itself (20odd yrs).
Each mirrored door is 50cm wide and floor-to-ceiling high; the mirrors
are stuck to the chip doors with 7? strips of double-sided foamy tape.
Those held long enough to avoid the installers being called back by
whichever previous owner had the work done; but as of today, all three
of the doors on the side which gets more sun, and two of the four on the
other side, have slipped down and needed regluing with Something Better.
Only two left to replace - and they're showing the first signs of
slipping down; fortunately, they don't detach suddenly, spectacularly,
and mirror-smashingly, but rather droop slowly off the doors.

So, there's an all-too-well-established ritual here - complete the
mirror detachment which time&gravity have set in place, take the door
off its piano hinges (thank the Pope for the light electric screwdriver
whose praises I sung here a few days back - there's forty-odd of the
little bleeders ;-), scrape off the remains of the dbl-sided tape. For
the door - Contiboard or similar - mere mechanical scraping is pretty
much all that's needed, as one can give it the requisite welly on a
surface which'll be hidden (and the odd scratch on will just improve the
keying of the replacement glue). The mirrors, though, have silvering you
really don't want to scratch off! So, I've tried a variety of solvents
on previous occaisions. The most effective has been a label-remover from
?Canford Audio? called At-Tack, but there's not much of that left in the
bottle... So, I got all systematic, trying
- meths
- the old grotty Dutch gin I'd already use on the doors
- WD40
- and the At-Tack
on the first 4 strips, leaving it to soak for a few minutes.
Surprisingly, the WD40 was the most effective, closely followed by the
precious At-Tack, while the two alcohols didn't soften and release the
adhesive much at all. The remaining 3 strips and the ones less well
removed by the alcohols came off well with the WD40; a final coating of
isopropyl alcohol and wiping up quickly with kitchen roll removed the
residue of WD40 well enough.

The adhesive I've used - today and on each of the four previous mirrors
- is 'Timebond', a thixotropic contact adhesive, which gives a tiny bit
of repositioning capability; 'tiny' because the combination of weight
and considerable surface area - a tad over 1sqm - means the mirror
doesn't exactly 'slide' over the door once it's lying on it, but a
couple millimeters of straightening up is possible.

And yes, I've had the windows wide open on both sides of the bedroom, to
let the glorious mix of: WD40, ethanol, methanol, At-Tack, isopropanol,
and toluene (the Timebond solvent) at least mostly disappear before
bedtime! Ahh, organic solvents [cue Homer Simpson noise... ;-]

Stefek