Since I had some of this around, I decided to experiment and see if it was useful. 
It is a steel-impregnated epoxy resin. You slice off what you need, knead it until the colors blend, then use within about a four-minute window of time. The stuff gets quite hot towards the end of the four minutes! It then hardens within the hour, turning dark gray.
I found that it didn't adhere well, both before and after drying. Sometimes it works for filling in seams, and sometimes it just falls off. I was able to use it successfully for one project, but there must be better choices.
I also tried it to fill in some pinholes on a resin model on my tabletop. It's too thick to easily work with, and doesn't adhere well. But if you can get it to stick, it works…
Costs around $4 USD and easily available (mine came from Walmart). Claims a three-year shelf life in the cappable tube.
Manufacturer's listing: link
They also make a version specifically for use on plastics.
learns to drill with a screwdriver.
