Tips on painting: WSF 3D prints are not ' fully dense' output objects. They are comprised of nylon with 26% air by volume, which is ideal for its lightness complimented by its super-toughness, and is suitable for all aspects of rocketry. The material's properties are akin to describing it as a 'nylon balsa'. It is stronger and more resilient than ABS, and certainly tougher than styrene. Strength to weight is much higher, thus less material is needed. Larger models with larger spans do flex, and general flexure is less when a finish coat is applied. The new PA12 is even better, fully solid nylon, so now its even stronger and stiffer! I found especially for WSF 'Elmers Color Change Wood Filler' that that turns white when dry, brushed on and sanded as the base coat filler , and the use of a spray primer over it, to be most efficient inexpensive method of providing a smooth base for paint. There is however a more expensive but very slick alternative in 'Smooth On' thin set epoxy especially made for 3D prints. Its what I do now for all my projects.One coat and your done no sanding needed if thinned down. You can also tint it with pigments! Great stuff!
As a reminder, all my designs are all hollow, including the fins in some subjects, for the lightest of possible designs to factor for stability. Look for other techniques that behave similarly for this approach and experiment. Have Fun!