Pigments and varnish?

[quaint violet]

I’m about to try out pigment powders, can I just apply them with a brush and then varnish over or should I varnish first?
Also, will any varnish do or should I buy varnish specific to miniature painting?

[low nacelle]

I usually apply then before varnishing. Also make sure to use airbrush medium to make sure the powder stays in place