Professional Gap Filling

You will have to try a few and see what you like.
I have the sqadron white putty and like it better than the green.
I have heard some people on this forum say that the Tamiya putty is great but I have never tried it. Still other's say that automotive spot putty is just as good as any of these and one tube will last a lifetime.
I actually use CA as a gap filler most often as it cures fast and is smoother than putty, although it can be a pain to sand sometimes as it is harder than putty.
Try a few out and see what you prefer would be my best advice.

Mike

You might also consider some of the "Plastic Welder" types of glues such as Tenax since they tend to eliminate a lot of seams in and of themselves.

I also use Squadron putty and Zap-A-Gap and have good luck with those. There is a post here: http://www.finescale.com/fsm/community/forum/topic.asp?TOPIC_ID=19822
discussing using some material called Porc-A-Filler for gap filling that seems to work very well. I plan to try it at some point, but haven't had an opportunity to do so yet.

For filling large holes you might take a look at Milliput. It is a two-part epoxy putty that does a good job of filling, but it dries hard as concrete.