Pens are not flat work or furniture. No need to use 120, 150 or anything less than 320. I can see scratches that were filled in some by CA.
The spots not filled in is because of using thin CA and applying it with Paper Towels. Thin is OK in many cases, but where grain and capillary indentations are at play, use medium or thick. If you are using paper towel, then you should apply more CA and in thicker layers. Remember, you paper towel is going to absorb at least 75% of your CA, so you are not getting nearly as much as you would think.
There are methods of applying CA so that 90% - 95% get on the pen.
I agree completely. I usually start with 400 grit unless the wood is far too proud of the bushings, then I might start with 320 grit. On my first grit I use a flat surfaced backer on my sandpaper (an acrylic blank) to help knock off any high spots without hitting the low spots too hard. I also always sand laterally after each grit to make sure all of the circular scratches are gone. I always sand with 400, 600, and 800 grits and depending on the wood, 1200 and occasionally even 2000 before applying a finish. The smoother the blank - the smoother the finish.
Some even reverse the lathe direction or flip the blanks between each grit to be more effective at sanding off any grains that were simply laid over from the previous sanding step. I have done this occasionally but it is not part of my usual regimen.
I do use paper towel as applicators in my regimen with lots of coats. As was stated above, I throw away more CA on my paper towel than I get on the blank. I apply 4 to 6 coats of thin followed by 12 to 14 coats of medium. For me this leaves the finish smooth enough that I can proceed right to Micromesh and buffing. I start with the thin because I think it penetrates more and almost acts like a primer to provide an improved adhesion layer for the medium coats. (No accelerator / activator on the thin coats and very sparingly on the medium ones).
I tried the more efficient use of CA methods on my way to my current regimen though. Some use the little plastic bags or other non-porous materials as applicators. The one that I used for quite some time was "art foam". It is essentially a thin, flexible, closed cell foam that doesn't absorb the CA. It did put on a much thicker coat per application, but I never could get the knack so that my coats were smooth enough to go directly to Micromesh when I was done. I always had to sand off some ridges in order to get the CA flat before going to Micromesh. Occasionally led to the dreaded "sand through" which is why I moved on. I'm sure if I would have stuck with it for a longer period I would have developed the necessary skill and dexterity to be successful with it.
Anyhow, I hope that gives you some additional insight and helps you develop your regimen.
Regards,
Dave