Hello Chaz. For filling seams such as you describe I use Bondo Spot and Glazing Putty.
What I've done is to cut the bottom of the tube off and squeeze it into a wide mouth jar. If kept in the tube, it will most likely dry out in a couple of weeks. Since this material is essentially lacquer primer with some kind of filler, probably talc or clay, It can easily be reconstituted by adding lacquer thinner. I have a pump bottle of lacquer thinner at my desk and add a splash about every week or so.
It cures through evaporation so works well enough for seams and other irregularities but not so much for larger areas as it tends to shrink and crack. I have had some luck building it up in thin layers smoothing each out as I go, but that's a slow process.
Hope this helps.
David G.