I use Humbrol enamel gloss. Mainly because I know it works and I use that for mini's anyway so I have some around. I have tried it with pva, which works but tends to be slower and not as tough. Poured deep and it sets only in places and leaves a most unattractive white sediment within the sealed bit, messy to remove too - like a pva sandwich.
If you're just thinking of pure varnish you can just pour it but don't go deeper than 2-3mm at a time because it will take an age to set, and can even ripple.
Here's one I did earlier
Just painted green in the vat and then topped off with varnish. Also if you go too thick, which I did with this one, in being a bit lazy with it, it will brown too - but I'm happy with the result.