Paints...

[bacteria]

You can get clear varnish in satin, matt or super gloss.

[Tibia]

Bacteria's right. You can get a clear coat in any finish.

Even though it won't make any difference on a black case, using an actual varnish or oil-based finish will turn yellow over time. A water-based or acrylic clearcoat will NOT turn yellow. It will stay clear.

[GoldenfrankO]

Thanks for all the answers guys. I'm going to test paint stuff, but my main reason for this topic is I'm finally starting my first case mod.