Hey Mark!
Squadron Green is a standard in the hobby, as well as Squadron White Putty. Both are of the same formula (basically the same as auto-body filler/spot putty), with the White seeming to be a bit superior. (That's only my opinion, however!)
They are both good for filling relatively large gaps, but not gaping holes. Both of them will shrink a bit when they dry, and they may pit some. This is normal, and a couple applications may be needed to fill a gap. If you have a LARGE gap, you may want to first take some plastic card or strip and stuff it in there with a bit of liquid glue. Once it's dried (and it should be absolutely solid) you can trim the excess plastic off and then fill any smaller remaining voids with CA or putty or Mr Surfacer or whatever.
Other fillers? There's a ton of 'em out there. From those developed specifically for modeling to general fillers such as the auto-body filler/spot putty I mentioned above. Another is a two part epoxy putty called Milliput. That stuff has a relatively short shelf life, though. A shame because I love the way it works.
I've used many of the fillers available, including the spot putty, and most of them work well. But my faves are Squadron White (which seems not to shrink quite as much as Green), Mr Surfacer, spot putty and CA. I picked up a tube of Gunze Mr Filler not too long ago, but haven't used it yet. I've heard from my model buds that it's great stuff, though.
You just have to try them to see if you like them. I love what I use, but they may not work well for someone else. My advice is to go for the tube of Squadron stuff. You really can't go too awful wrong with it.
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