Sanding and filling tough, small, weird spots?

cheap battery operated electric toothbrush. cut the bristles off, glue on a dowel and sandpaper on the end with  doublestick tape. or just sandpaper strips, secure the model in a vice and "saw" the sand paper left and right like doing a gun barrel.

Thanks everybody, I will definnitely try everything you have recommended. I probably need to take a look at some needle files myself, as they iften come up when talking about this.

Anytime I get putty in a weird spot. I do one of two things, or both. A q-tip dipped in nail polish remover (acetone) and I'll "rub down" the putty to where I need it. The acetone dissolves it at just the right rate. I find this also works especially well on wing root connections. Or, I'll take a tiny bit of sandpaper or whatever grade I need and super glue it to a toothpick and go from there.

My needle files do a very fine job that ordinarily doesn't need further sanding.  That is the beauty of fine files- they take off excess like a medium grit, but don't make noticable scratches.  And, you can clean the styrene or filler dust out of them so they last a long time.

If you have any scrap plastic that mimics the shape you are trying to create, you can make a custom sanding tool by gluing on sandpaper or wet & dry paper with rubber cement. I use needle files to get to the rough profile, then finish up with these custom "sanding blocks".

Yup. I would try sanding needles found in Hobby Lobby. They come in coarse, medium, and fine grit.

I picked up some sanding sticks at Hobby Lobby a while back that work well in spots like that.

These aren't the same brand but they look just like this:

For situations like that I use one of two methods. I have a set of needle files. I consider these a necessity. I would use the round cross section file, that is tapered and comes to a fine point.

Also, most fillers can be carved by a good, sharp hobby knife blade once they are fully dried/cured.  In cases like yours, I have carved away excess with the hobby knife and then finished up with the needle file.

I have also used my Dremel tool with a very small ball end.  My battery Dremel has a speed adjustment that goes down to a very slow speed.  However, I would avoid that in a case like you show, as in a confined space if I touch one of the surfaces near a corner the tool tends to bounce around, making nasty scratches and marks.