I’ve found another great product to use as a filler: Gorilla super glue.
    You need very little of this product to fill gaps, because as it dries it also expands. For normal gaps you may find in a well-designed model (such as wing or fuselage seams), I actually apply a very small amount to the seam with a toothpick.
    The super glue expands and actually overfills the gap. Here is the neat part about using this as a filler: If you wait 1-2 hours (the no longer), the glue is still rather soft but firm; you can use a hobby knife to slice off the excess flush with the surface – and you are done. No sanding and rescribing lost panel lines. Also, the stuff dries to a white color, so it is easy to see.
    If you really feel the need, you can crease a piece of wet/dry sand paper in half and use the very narrow face of the paper to touch up the seam. No muss, no fuss: Give it a try!

Tip submitted by:
Jim AllmanGrayling, Michigan