Here is a tip for gundam modelers that would make the plane and tank guys cringe in pain. Not a new trick, but some of you might not know it.
Cut the parts to be joined of the sprue and clean as normal, next take a set of cutters and remove those huge alignement pins, sand smooth.
Now you are ready to go, take some glue and apply it to the joint halves and set aside for a bit.
When the plastic has started to soften mate the halves and move them around a but(rub them against each other using a bit of force), this should force some plastic to come out along the joint. Aligh the parts correctly and set aside to let dry.
Once dried cut/scrape the small ridge across the joint off and sand smooth, voila there should be no joint or hardly any visible.
One way of joining and filling gaps, not recommended for models with lots of surface detail.
I actually use this method a lot especially on kits where I don't want any alignement points visible i.e. inside of Bike exhausts, etc (here in addition I sand everything smooth on the inside so that only the part walls remain).
Nothing worse than seeing a kit that looks nice but as soon as you move a joint the alignement point sticks out like anything.