Sounds like you have NOT built a model yet. Most rocketeers are also "expert" modelers and "require" perfection in terms of look. STEM project- not sure perfection for looks is required. My 1st few kits I did not used primer. After I read most modelers do, I prime but I may or may not sand it off. I filled my Balsa with plan Delta Cream Coat all purpose filler which no matter how many times you apply, you still have grains. I will try DAP plastic wood, filler since Home Depot seems to no longer carry Elmer's here in NJ, I used Elmer's once and it gave me trouble, keep falling off. And you are right until this year, I never saw sandpaper in "grit specifications" at Home Depot, just coarse, medium, fine and extra fine and that is what I used.
So do not become obsessives with trying to get an expert appearance on tis project....it is the science behind it, a STEM Project. Use the best skills that you have obtain during your lifetime. If it was perfect, then any aduilt would say, Yeah his father built it.