I'm not sure that it would be worth it spraying the shopline primer over top of the Akzo primer. Either way, you will want to scuff the Akzo, but once it's scuffed I'd think that there would be only a small difference between spraying over the Akzo vs. the Shopline, and it would add some weight. On my RV-10 I did spray some paint over Akzo and it seemed to be ok. I'm no paint expert by any measure...I've only done this twice.
The biggest things are scuffing and clean/degreasing. Preparation is the hardest part of the job.
Also, in a way it's hard to argue with Mark's follow-up comment about using rattle can paint, from the standpoint of being able to quickly paint additional parts. When I've done that, I've added parts and used some rattle can primer of similar color and it's convenient, but you will never get a perfect match. Fortunately, for the interior there's likely to be little metal you'll add later, more likely fabric/leather/vinyl. But I do like the durability of the regular auto paint, and I know the 2-parts are often more resistant to chemicals.
A few people have asked what paint my interior is and I said I wasn't positive but thought FDG (not FDGH). So I took a photo last night.
As Mark said, you may want to etch prime the interior. (I would for sure if using rattle can, and they make an aluminum etching primer in rattle can style) These days it seems that the paint shops that use/sell auto paint though, are leaning away from using an etch primer and leaning more towards an epoxy primer as being suitable enough for aluminum. I used etch primer on my exterior aluminum, but I'm not sure what I would do next time.
On the interior, I used a cheap epoxy shopline primer. But on the exterior, my fiberglass parts (other than the wheel and leg fairings) were first primed with the PPG Epoxy Primer. Here is a photo:
The thing is, I don't know if you really need the epoxy primer on the fiberglass. It's a good sealing primer, and you can also just use K36 primer/surfacer. And on the aluminum you can either use etch OR epoxy. The epoxy isn't meant to be a very sandable primer. So for exterior you can either Epoxy + K36 + Paint on fiberglass, or just K36 + Paint. And on the aluminum you can either Epoxy + Seal + paint, or Etch + Seal + Paint, or perhaps even Epoxy + Paint. The methods and options are hard to navigate, but my guess is that you can have good results with most any method.
Again, the preparation is the most critical to getting things to turn out well.
Check out these pinholes in my intersection fairings. I'm thrilled to say that I finished the wheel/leg/intersection fairings this weekend and have them all installed so nothing more to build! The prep work on the legs/wheels was worse than any other fiberglass part on the kit.
All in all, it took me maybe a week or a little more to get them completed done, so it wasn't the end of the world. It helps if you don't have "showplane" as your personal standard, I'll tell you that much. I'm just happy to have a decent looking plane. If you come to OSH, please stand at least 1 wingspan away from my plane for best viewing.
That's what you get for amateur work from someone who cares more about flying and long term maintenance than show quality. I really do feel good about the "did it myself" aspect though. This airplane had absolutely no professional help other than the seats and stick boots, and the Koger Sunshade which I wouldn't be without.