Duro and other adhesive makers produce 2-part clear epoxies. I mentioned Duro because their's is in the "optically clear" range. Some have a slightly yellowed look. Each usually comes in a clear blister-type package and the color is visible. After painting detail - dry-brushing,etc - I mix equal amounts of the epoxy and touch an appropriate amount to each instrument with the tip of a hobby knife #11 blade. It may take a few touches to fill an instrument depending on size but properly done the instruments looks to be glazed. It's a lost effect to a large extent (closed cockpits or smaller scales) but can greatly enhance otherwise. On the giant scale flying birds I've built (1/4 scale) it's a must. It also works well for exterior lights such as landing and marker lights where there's a depth to be simulated. On the red, green and amber lights a touch of translucent color (not opaque) in the epoxy does the trick nicely. Being epoxy it's inert, won't dissolve plastic or paint and can be tinted with enamel to good effect. Food for thought.