Welcome to the forum, Huey!
Like Don stated, there are about as many methods of seam-filling as there are people in the world. LOL! As Tojo said, if you can get your hands on some Mr. Surfacer 500, that's some pretty good stuff. it's great for smaller seams. In lieu of that, I like to use plain old White Out correction fluid. They're both roughly the same consistency and both are sandable and dry quickly.
For larger seams, I like to use Tamiya white putty. It's sort of like applying white toothpaste.
Some people use thick or "gap filling" super glue. I've used this a time or two but I think I like putties the best. The super glue will sand and it'll definitely fill the seams but it's been my experience that I've got to start sanding and working with it within about 15 minutes of applying it. If I wait longer than that, the super glue will cure harder than the surrounding plastic. You'll end up sanding away more of the surrounding plastic than you will the rock-hard super glue. Again, it works wonderfully if you stay on top of it. I tend to apply putties and then walk away and forget about it for the night. I can't do that with super glue.
I hope this helps and, once again, welcome to the forum!
Eric