Bead blast the inside of the ashtray and then rub it with 0000 steel wool. That will take away the blasted finish and leave a somewhat shiny metal finish. You could just mist a clear coat over it but it may start rusting again. It just depends on the climate.
If you havn't heard about it, there is a company called ECS and they make a product for preserving bare metal componants. It's called RPM and it's a paste. You heat the part you want to coat up to 120 degrees, then rub on the RPM. It turns to a liquid and seals the part. Once it's dry, it is invisable. That would be the perfect product to use on the ash tray.
You can also send the tray in to be replated. It won't hide the pits on the inside but it will keep them from rusting again and the outside will look like new. You can use the RPM over the plating too if you want.