No, please do not apply the backing before painting. This is because all handmade xuan papers which are made with high percentage natural fibres ‚breath‘ too much when you are painting. Therefore please apply the backing after your painting is dry. This will ensure that the mounted painting is flat and the colours in the painting can also be enhanced.
The best adhesive is the one you made in your kitchen. We have shared the recipe here,
Mounting Materials and Tools
Glue: it is made with flour and alum. Also, in order to ward off mould etc, you can also add some antibiotic agent in the flour when making the glue.
And always bear in mind: the best materials for Chinese painting is always natural. This will help preserve the painting for centuries and will allow later restoration.