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 perfect for its lightness and super-toughness, and is suitable for all aspects of rocketry. The material's properties are akin to describing it as a 'nylon balsa', and by no means soft. Stronger and more resilient than ABS, and certainly tougher than styrene. Strength to weight is much higher, thus less material is needed. With my larger models with larger spans of surface area, yes, there is subtle flexure as a positive attribute.
I found enamel paint as the base coat or primer to be most efficient method of finishing these parts. All my designs are all hollow, including the fins, for the lightest of possible designs to factor for stability. Look for other paint types that behave similarly for this approach, or do your own experimentation.