On Wed, 28 Oct 2020 18:26:50 +0000 (GMT), Dave Plowman (News) wrote:

I don't think it's a mortar based adhesive. Not the sort you'd use for
tiling outdoors. More water resistant than waterproof. Fine for an
occasional use shower room walls.

The packaging will tell you. Or just keep it wet for an hour or two
ordinary grout/adhesive will soften, waterproof won't even notice.

Thin film may come off with a non-scratch scourer.

--
Cheers
Dave.