shrike,
It seems as though the others have misinterpreted your request for help. To help hide, or prevent, seams, I use Tenax 7R on the fuselage joints & wing-half joints. When used as indicated on the bottle, you'll get a raised bead of melted plastic along the seam that is very easily sanded off, leaving NO SEAM!! I understand that are other adhesives and techniques with which to accomplish this, but this is what I use. Your picture doesn't readily show the mistakes that you are asking about, so maybe that's why you're not getting the responses that you're looking for. The picture really does look great, by the way!!
As far as the wing root joint goes, once you have made sure that you have a STRONG bond for that joint, apply some putty (I use Squadron Green Putty, but I understand that other brands will work also) to the joint, and, after about 60 -120 seconds, wipe away the excess with a Q-Tip soaked in either alcohol or nail-polish remover (I've used both with no problems). You SHOULD wind up with a seam filled with with a very smooth application of putty that doesn't need any sanding. I forgot to mention that applying tape along either side of the joint before you apply the putty will help to keep unwanted excess putty from getting to other areas.
To mask a canopy, regular old masking tape will do the trick. Just cut some very narrow strips of it from your roll, then carefully apply the tape to the areas that you want no paint to touch. This technique is VERY time-consuming compared to using Bare-Metal Foil, but it works just as well. Just make sure that you hold your masked canopy near a light to make sure that it's correctly covered before you paint.
To get a good demarcation line between the upper and lower paint colors, you really do need an airbrush. I'm sure that it can be done with hand-brushing, but I could never accomplish it. Maybe someone else out there in modeling land can help with this item??
Anyway, I hope that this information has helped you out some. Good luck in your modeling endeavors!!