I know exactly what you're talking about. I had the same issue trying to get the fin fillets smoothed out with constant sanding and filling even though they are paper. You typically get a bit of glue here and there, sanding scratches or sanding so much that I began to delaminate the tube itself. A lot of crap I had to fix or dress up because of being overly rambunctious. Just out of curiosity, what exactly are you doing with sanding primer? What grades of paper are you using, wet or dry sanding, any filling in between, what filler if any, and for that matter, what is the size of the rocket too?
These and other factors can make or break the patience of anyone's will to carry on. My bi-polar disorder is my biggest enemy causing all sorts of grief and will to carry on. So I sympathize with anyone who is running into a lack of interest in anything. I think it goes without saying that you have the support of everyone here to get that bastich project of yours under control. :wink: