The obvious sometimes needs to be restated from time to time. If you concentrate on the basics, the end results will be better. So it goes for seams too. Do the proper prep work and you'll have less work...seam filling. But when you do your best and still have a seam well, that is the whole point of this thread...how to read them and what to do about them.
Filling resources are:
Some modelers don't even consider using styrene as a filler. It does take a little effort but if you ask an old body man, he'll tell you metal, there's no replacement for it. So it goes for styrene as well. When you use styrene, you get a strong bond made of similar material. CA works as a good filler but it has its own little set of rules. You must work with it fast before it fully cures, if you wait you'll be sanding away at a rock hard material surrounded by softer real estate. Not necessarily a good thing and much more work.
Putties, well everyone has their preferences, as I have mine. I've probably tried every kind out there including window glazing...and yes it does work. When I need a filler putty, my preferred choices are 3M and Squadron. Regardless of what you use, use it sparingly. Glopping on loads of putty will only create more work. Several thin applications are best.
If you have any specific questions just ask. I'll see if I can provide the answer. More to follow...