I use green stuff, which is just milliput by another name. It goes quite flexible so I roll it into very thin sausage shapes, wedge them into the crack then use a cocktail stick or similar to work it in. Add some water at this stage as it really helps to smooth it out, get it as smooth as possible then let it dry. You can sand it and trim it once dry.
I do sometimes also use fine metal dust from filling down a model, I use superglue then sprinkle the dust on it.
Since milliput and other typical modeling solutions are not readily available here, I have found a white adhesive at Hobby Lobby that is really for ceramic tiles fill in the gaps nicely and dries a clear color but holds primer and paint well. The Forged In Battle split waist Arab camelry do not fit flush and left a significant gap in several places once glued together. I discovered the adhesive would work while playing around with the materials I had on hand and was pleasantly surprised with the results. It also has the added effect of providing a bit more grip to the epoxy and super glue used to mesh the two halves together.