Varnish over bare aluminium.

[Kesh]

Have a few questions about this.

1. What's the best varnish to use? I generally like using spray-can clearcoat, as it's easy, but I usually use this over paint on primed aluminium.

2. Will it dull and tarnish very quickly?

[Pyr0]

i've used automotive acrylic with good success.
The first coat or two needs to be very light or it will run and drip down the sides, normally the primer/paint help hold the lacquer.
if you polished the box to a mirror finish it will drip even more.

[Toney]

 Finely sand the surface.
Clean with alcohol, not touching the surface.
Spray with mag wheel clear coat. Its extremely robust and designed to go over polished metals.

Thanks to Darron for this idea.

[Sacorus]

Clean surface with shellite, then hand apply satin poly.