What is the purpose of the primer if your not sanding out imperfections? If you don't mind me asking?
I'm not convinced that the purpose of primer is specifically to sand out imperfections. That would be true for filler/primer, but not the regular stuff. Anyway, I have two separate circumstances in mind:
1) My upscale Quinstar. This is a mixture of bare balsa, basswood, and papered parts. Although I painted my regular Quinstar as recommended by JumpJet (no primer, just a few coats of paint, for lightness), I believe this one needs some primer to ensure a consistent topcoat given the varied surfaces. I also note that Rustoleum (I think?) specifically recommends primer when painting on bare wood. I wasn't planning to sand the primer; there are many nooks and crannies in that crazy thing that are nigh unsandable. There are a few areas that could be sanded, conceivably, but certainly not the whole thing. Anyway, if I'm *not* going to sand, then it would be nice to do two light coats of primer and then go right to the topcoat. If that works.
2) Starship Avalon (see my avatar). This will have a black basecoat followed by a set of fluorescent green highlights. For the green, then, I must mask the heck out of the rocket, then I will spray a coat of clear to (I hope!) seal the masking tape, then one or two light coats of white (not necessarily primer, maybe just regular white paint) then the green. In this instance, there is *absolutely* no sanding during this sequence. If I'm disciplined, I believe I could do all the necessary coats within an hour. Alternatively, I'll need to do the clear and the white, then wait at least a week or until next painting opportunity to do the green. I'd prefer to get it done all at once. If that works.
Note: I have thought about it a lot, and it is not practical to do the green before the black on the Avalon, in keeping with the theory of starting with the light colors and finishing with the darker. I have done a test and a layer of white followed by a couple coats of green looks great on top of the black. Wait, I have a picture of that somewhere.... ah yes here it is. It's very messy because it's all done by hand, but it proves the point.