Super Glue

I personally like Zap brand better than any other but there are people who like other brands as well. I would not recommend the Super Glue that Crazy Glue produces as it isn't very good in my opinion.
I like the Zap CA+ with the green label as well as the thinner, faster Zap in the pink label.
You just put a few drops in a bottlecap or similar small container and then use a drafting pen, micro-brush or toothpick, and pick up some of the super glue which we call "CA" for short (Cyanoacrylate), and put it on the gap or area you want to fill. Let it sit until it hardens and then sand it. The longer you wait the harder the CA glue becomes and the tougher it is to sand.
Some people use CA accelerator also as it cures the CA glue instantly, but I have found that it tends to make the CA harder than just letting it sit for 5-10 minutes and then sanding it.

Mike

If you sprinkle a little baking soda on super glue used as a filler, it is easier to sand later. I buy the cheapest possible, like the house brand at my local hardware store, it even has an applicator brush for discriminate application.

Mike is right on the mark and clfesmire has a great tip as well. Let me expand on clfesmire's tip a little if I may. If you pack the dry baking soda into large gaps you can then add the thin type CA to get a perfect fill. The dry powder will wick the CA into it's self. If you add the powder after it will still work but clean up will be more extensive.

Thanks for the info. Every little bit helps.

Actually superglue can be better than putty, since it does not crack or swell or produce dust. Even the usual home purpose superglue is fine....