Here's my top tips for fitting wings to fuselages.
First test fit everything before you glue it together.
If you have a very large gap it may be possible to 'splay' the fuselage slightly to minimise this, but there are limits imposed by what's in the fuselage, usually a cockpit assembly. You can use bits of sprue across the fuselage to hold it out, just make sure that your 'spacers' won't interfere with the fit of anything else!
Also make sure that you have the wing dihedral (the amount the wing bends up relative to the fuselage) correct. It may be you need to lift the wing tips which will close the gap at the wing root. Don't go mad with this. I've seen models with nice wing roots but rather improbable dihedral angles
Anything but the largest gaps, which may require some plastic card shims, I fill with CA glue (superglue). I run a medium or thick CA into the gap and immediately sprinkle either baking powder or talcum powder onto the glue. The talc is for the smaller gaps and has the added bonus of smelling nice. Leave it thirty seconds and sand it flat. Don't leave it for any length of time as this home made filler will cure much harder than the surrounding plastic and become difficult to sand without causing precisely the kind of damage to the surrounding area which you are trying to avoid. Unlike plastic putties it doesn't take hours to harden and, crucially, it doesn't shrink. I estimate that I can do a wing root in less than five minutes.
Small gaps can be fixed with your normal plastic filler. I swipe it in the gap and remove any excess with an cotton bud moistened with an acetone based nail varnish remover. If you are careful you won't need to sand at all, but you may need to repeat the process due to aforementioned shrinkage.
If your test fit reveals a step rather than a gap then you need to think about ignoring the instructions. Steps are very difficult to fix seamlessly and without losing a lot of detail.
A good trick is to fit the upper wings to the fuselage first. This should leave no gap (obviously) and also no step. When you come to fix the lower wings you may need to make some adjustments but the most usual result of this technique is a small gap along the inboard leading edge of the wing which is easy to fix and, most importantly, easy to fix invisibly.
Others will have their own techniques, but many will be variations on this basic theme. There's almost always a judgement call to be made about how to go about solving each problem and that will only get better with practice and experience. I've been making model aeroplanes, joining wings to fuselages, for a long time and I still learn from every model.
The most important thing is to have fun, try things out and see how they go. Every model you build is not going to be a masterpiece........ask me how I know
Cheers
Steve
