A.T. wrote

I have now stripped it back to bear wood and I'm about to repaint. I don't
want it to happen again.

Loss of adhesion in this form really only occurs when something is
trying to escape from the substrate through the paint coating film. This
is usually moisture, but can also be natural resins and/or solvents
still present after treatment.

The fact that you have noticed a relationship between the mirror
position and the shelf looks like a reasonable area for investigation.
It's difficult to diagnose without seeing the problem, but I would guess
that the problem is moisture related, and that condensation could
possibly be forming on the mirror and running off directly onto the
shelf. Constant exposure to water lying on the horizontal shelf will
shorten the effective life of the coating.

However if the shelf is fitted flush with the wall, its more likely that
condensation is running down the mirror and either down the wall or
dripping onto the back of the shelf, and the rear of the shelf (where it
is fitted to the wall) has not been properly painted/sealed. Moisture is
probably getting into the timber at this point, then trying to escape...
with the consequent loss of adhesion you have seen.

Make sure the shelf is properly painted all over, even the parts that
you can't see... if necessary remove from the wall and paint. You could
also run a thin bead of white sanitary silicone along the rear of the
shelf where it joins the wall for additional protection.

Regards
--
Max Bone Decorating Direct Ltd
http://www.decoratingdirect.co.uk/
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