For smaller seams, I use Liquitex Acrylic Gel Medium...it's got the consistency of toothpaste, so it's easier to handle than white glue. It dries clear and hard, but it's flexible to a degree. Best part is, it cleans up with water, so when filling a seam, I let it dry partially, then clean the excess with a Q-tip soaked in water. This prevents me from sanding off the details, like planking, etc.
Gel Medium is available in any art store, about $12 a tube.
Jeff Herne