urgent advised needed on protecting outdoor mosiac project

Hi there,

Im pretty new to the whole mosiacing thing, but recently started an art job which required me to continue an outdoor project commissioned for a local childrens centre.

We have been creating mosiacs, using 8mm ply, no more nails to fix the mirror and tiles, water proof grout and a grout sealer. the finished pieces will then be fixed to a larger 18mm ply board surounded by more grout, and screwed to the wall, which is outdoors, but in a semi covered area.

My concern is will this be enough to prevent the grout eroding over time, and or the tiles falling off.

The covered area is above an area for the parents to store their buggies, so if even one of the tiles were to fall off into them it could have really serious consequences.

There have been a few raised eyebrows at the use of no more nails, will this be strong enough to hold over time.

ALSO we've had some trouble getting the grout to stick to the sides of the ply, is this because it is more pourous and therefore sucking the moisture out of the grout? is there anyway to prime the boards prior to grouting? And if not does it matter if the whole piece is not covered and if it does, could we maybe varnish the sides?)

Really should have thought this through a bit more before starting, a lot of people have worked on this piece, so it would be a real shame to have to scrap it, any advice would be graciously received!

Kind regards loud!