If you are going to be working with prop kits or scratch building replicas, you should get to know this material!
It can be found in the plumbing supplies aisle at Home Depot. It's in a plastic tube about 8” long and 3/4” diameter.
You cut off a section of it and knead it in your hands until the grey part and the white part are totally mixed together. At that point you've only got 5-10 minutes to work with it before it's as hard as a rock, so don't mix up too much at once.
It bonds well to just about anything, so it's great for repairing a chipped resin piece, building up small details, filling holes, etc…
You can shape it with your fingers to get it relatively smooth and free of any gaps or bubbles, then sand it smooth.
This putty stuff is one of the best materials I know of for small fixes. It doesn't smell all that bad, is pretty easy to work with, and cleanup isn't terrible. That being said, you may want to wear nitrile gloves when you are working with it, since it will leave residue on your hands.