Sanding and filling tough, small, weird spots?

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.

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:

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

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".

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.

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.

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.

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.