Ever come across the issue of a molded part with the back open, leaving a cavity empty that just should not be there. A normal resolution would be to cut a piece of styrene to fit, then glue it in place. This meant some measuring, cutting, test fit, sanding, and so on.
Well I keep a bottle of Deluxe Rocket Powder on my desk. To fill a small to medium cavity, it is just a matter of filling the cavity with the Rocket Powder and adding a drop or two of thin CA. Since it may be difficult to get a flat even surface, I fill the cavity up leaving a small gap with the top of the cavity and after the CA dries, I use some putty(I use Tamiya) to complete the job, with a quick sand at the end.
Presto, cavity filled.