As I go into my 44th year of my kit car building hobby (obsession???) all I can say is.... boy, are you going to be busy.... You've got a pretty typical "new" kit car body there. That white stuff is probably a type of poly surfacing agent that is shot over kit bodies to make them look less terrifying to the customer. I almost wish manufacturing concerns would quit doing this so you can readily see the flashing seams that must all be addressed long before paint work is a consideration. All of the individual panels are done and while still raw put into the larger sectional molds then "flashed" together. While this process seems to make sense what you normally get within about 2 inches on either side of any seams are areas of of air voids, bubbles, and ragged glass. That heavy white coat hides all that.
I'd be inclined to sand the white stuff off a couple of test areas on seams (both inside and out) and see what things look like. If it's looking "fishy" then I'd be opening those seam areas up everywhere to re-dress them. They would best be roughed, picked open, etc., then filled with whatever resin your kit manufacturer has used or recommends. Another option is to use high strength epoxy resin IF you think those seams need to be stronger to prevent sag/flex. Those are awfully large body pieces you've got there so that could be a factor.
As far as sealing/priming here is just an opinion. With the generally crappy quality of most fiberglass now.... I cut all of the openings first..... I do a guide coat over all the white as the car sits RIGHT NOW, yes, now.... why?.... So I can take pictures of the sanded guide coat to see just what I am up against with body work, waves, etc. Those pictures are my reference for later. Next I mount ALL of the body panels as they will be done when the car is finished.... again, why????.... because you need to work out the NVH (noise, vibration, harshness) now before you get anymore coating on that car. Next I get all of the seam work done, get any major body work done (waves can wait till later) and surprise!.... I seal the whole thing up with epoxy and drive it for 6 months. Yes, I drive it for 6 months looking and checking every panel for movement, subsequent cracking, and just about anything else that I think is going to screw up my great paint job later. I even have my wife or daughter pace me with cameras/videos as the car moves looking for problem areas. Countless times I have found that I had to build sub frame structures to support vibrating or worse flapping panels. Better to get that stuff under control right now....
This might be of some help to you....
viewtopic.php?f=12&t=7938&p=48940&hilit=Darrelk+fiberglass+kit+build#p48940That was my last full kit build. I don't think yours will be too far off from the same general principles I used there. Bottom line.... just make sure that body is absolutely solid mounted and "exercised" before you begin any fine tuning of the body work and paint process.
Oh, and just curious, what's your base.... Fiero, Mr2, Vette, Porsche?????