Sanding pin marks

Sanding sticks,and use the finest grit you can get away with.very easy to over do it

I find it's easier to fill them with a little putty and let that harden and then sand down the putty very carefully with a pointed sanding stick than to actually try to eliminate the indentation by sanding down the edges of the indent. I use a tooth pick to apply the putty and push it into the indent and then roll the tooth pick over it if possible to smooth it down some. The hardened putty is softer than the plastic so it sands down easier with a sanding stick point than trying to sand down the plastic. And in those tight spaces, you don't need for the surface to be perfectly smooth. You just need to eliminate the indent and get the surface somewhat level.

If recessed marks, I fill with putty and then file smooth. If protruding, I file them with needle files.  I like the needle files for this work because it takes off plastic as fast as coarse sandpaper, but leaves very fine scratches like quite fine wet or dry paper.  Also, files can be cleaned many times, so I find them cheaper than sandpaper.

BTW, there is a Techniques forum in the FSM hierarchy where seam filling and ejector marks are a frequent thread.

cheap electric toothbrush, cut off the bristles, glue a dowel on the end and attach sandpaper  

I'm working on an old 1/72nd T-33 for my 5th F.I.S. collection and have run into a couple of those.  There's a couple things you can do here.  On a flat surface I sometimes use layers of superglue to fill in the hole.  For others as well as the round areas I usually use Tamiya's white putty.  When it's dry you can use a cut piece of wet n' dry sand paper to lightly go over the spot or a flex file.  If you haven't seen these, it's a "u" shaped piece of  aluminum that takes a strip of sanding material connected to each end.  You can buy extra grades of roughness as you need them..