Is the Daler Rowney soluble varnish suitable for outdoor use?
It is not always obvious whether a varnish suitable for out door use. Many acrylic varnishes along with acrylic paint are tough enough to be used outdoors.
I would only choose one which specifically says suitable for outdoor use or even one from a DIY shop like a yacht varnish which will protect from weather and offer UVA.
Here are some varnishes we stock which do say they are for outdoor use:
Golden MSA Varnish: Available in Matt, Gloss and Satin