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:
Now I've got one of two panels on. A little bit of sanding will be needed but not much.
This is the outline of the inner panel. Notice one corner has a larger radius.
And these are the panels themselves. They'll simply be glued to the outer panels.
And now with the inner panel glued on.
This is the beginning of the riveting. I'm just using watered down Titebond in a syringe. 
Will be a long road here folks.
And now the icing. I use Red Devil lightweight spackling. This stuff sands easily. After this is done. I'll finish off with the riveting.
I got the 1/16th angle from Plastruct today. These will be placed just over the access panels in a curved shape as the photos on page eleven of the real fin can show.
