For filling gaps, I like baking soda soaked in thin super glue. Using a putty blade, I trowel the baking soda into the gap, brush off the excess, then apply the glue. Do not use accelerator. The baking soda will harden quickly, and the seam can be sanded after just a few minutes. It sands smooth, is tough, and it takes paint very well.
Tip submitted by:Mark AlerioNorth Andover, Massachusetts