If you're having problems with putty (like me) You could use elmer's glue as substitute.
Here's how I use it::-[ ]

1. Combine the two parts first with a superglue
2. Fill the gaps with elmer's glue
3. Press the glue to seep it in
4. Remove the excess glue
5. Let it dry
6. Sand it down
7. Wash it with water (Little amounts only). This serve as double purpose:
7.1 Remove the dust from sanding.
7.2 Elmer's glue is removable with water. This property will let the glue to liquify and to seep in other small gaps
8. Let it dry
9. Now there will be relatively smaller gaps.
10. Now repeat the whol process again until the gaps are entirely closed.

Believe me it works for me!

Nice tip. I'll have to give it a try. It sounds like a good alternative.

I use Elmer's all the time, for small seams in places that would be next to impossible to clean up if I used putty. I saw an article in FSM where a guy was using it around gun tubes on a 1/700 scale battleship. Works pretty good.

I also use elmers glue to fill some gaps. Another tip is after application the excess can be removed while still "wet with a damp q-tip or rag. Sometimes if you get lucky it comes out smooth enough where sanding isn't even necessary.
One other propery you forgot to mention is it also dries clear, so if there is a gap found after you have painted the parts. The glue carries the pigmant of the paint so touch up sometimes isn't necessary.

I've tried it since putty is more expensive and I found it hard to use. Enjoy!

Believe it or not, you can use white-out correction fluid using the same technique. Works pretty good.

The best stuff ever for this purpose though, is Gunze Sangyos' "Mr Surfacer." It is putty thinned to the consistency of thick paint, and it is awesome stuff!!!!

Scott