Plumbers' putty is definitely not the right choice for that. It's designed to remain soft for a long time, which I doubt is what you want. I don't know what you have to fix, but a two-part epoxy would probably be the way I'd go. Just mix a little up in a dispoable cup and use a toothpick or something like that to apply small amounts of it to what you want to repair. It won't be the prettiest looking thing, but it holds very well in my experience.