How do you fill and sand seams on rocket nozzles that come in halves and have raised piping jackes around them.
With great care 
I have a set of needle files I use for cleaning up putties on difficult seams. I find a triangular needle file reaches down into the notches on that kind of nozzle fairly well. It is still a lot of work, since you have to do each notch individually, and that will teach patience or character, or else help you decide modeling is not such a relaxing hobby
Just kidding, you’ll get used to it. Hopefully the model you are building doesn’t have too many nozzles!
Important that when you putty or fill a seam, mask very carefully and closely to the seam itself.
Push the tape down in the tubing with the edge of a knife. Leave maybe an 1/8" wide exposed area. I spend more time in prep than in cleanup, but it’s worth it.
I would love to see video on the tools/techniques you guys are describing.