So annoyed with myself.

Thank you, MrsKen33 and Esmay
for your ideas.
They are both characters I’ve never heard of, so can’t remember the names! One is a cat and the other is a bear. My daughter is going to put the fine detail on them - I’m just making the shell!

I've made lots of masks using papier mache on balloons and never used salt .

If you varnish the mask - no mould could escape anyway .
Having written that I've only vanished the outside and not the inside !
So I'd apply -
more papier mache with salt to the inside and vanish the outside .
And stop worrying !

We made loads of those at school and I can never remember using salt. Hope it turns out well, and what might your mask be?