I watched this video from AwesomePaintJob.com a while back and had wondered how well it would work.
The premise is really simple - put down a layer of superglue in your gap, cover it with baking soda, then file it smooth once it has dried. It seemed to simple and I was a bit skeptical so I was pretty happy when I had a small gap to fill on my Imperial Guard Manticore conversion that I am working on.
Here is the gap. It was between two pieces of plasticard that I had cut to build a platform for the Manticore rocket launcher.
I laid down a thin line of superglue across the gap.
I then spooned a liberal amount of baking soda on the glue and let it dry for just a bit.
Here it is after I shook off the excess.
And here is the final result un-primered. It is pretty smooth and actually did a really good job of filing the gap.
Where this method really shines is after you have primed the model. You can see in the picture above that you really can't tell there was a gap there without really looking at it. Once the model is fully painted I doubt you will be able to notice at all. This is a very handy trick to have in your bag of modeling tricks.