Bondo spot putty is essentially talc in a chemical solvent for PAINT. It will bond better to a paint subsurface than a raw surface, I'm told, due to the uniform chemical surface of the softer paint than the harder plastic. True or not, actual cause or not, it seems to bear out in practice. It'll blend better when sanded, too, if applied over primer. Since Bondo is a car product, I'll trust the car guys to know how to use it best, and they all say primer, then spot putty.
Squadron putty is essentially liquefied poly styrene, and thus is chemically most suited to model type plastics as it's chemical solvent is the same as the base material. It'll attach to paint well, too.
Most plastic LP/MP nose cones are Poly Styrene, and generally get surface dissolved well by the solvents in Bondo spot putty and especially well by the purpose made Squadron putty. Thus, the general practice is putty then primer.
Most HP nose cones are Polypropylene or fiberglass, and not overly affected by solvents in general or at all (thus the common usage of adhesion promoter when painting them), so the general practice is primer then putty.
So, the order of application depends on the base material.