Tips on painting: 3D prints of this kind 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. I found 'Elmers Color Change Wood Filler' that that turns whiite when dry, brushed on and sanded as the base coat filler, and the use of a spray primer over it, to be most efficient method of providing a smooth base for paint. All my designs are all hollow, including the fins, for the lightest of possible designs to factor for stability. Look for other techniques that behave similarly for this approach, to do your own experimentation.