Some time ago I discovered a quick way of filling small gaps quickly without having to dig out the Green Stuff. It basically involves creating some plastic shavings, dipping them in liquid poly cement and applying the resulting 'goo' to the area to be filled. Check it out.
You will need:-
An old paint brush, a GW 'standard' size brush is about the right size. Mark the brush with a bit of tape so you know it's only for glue. Dont worry too much if after use the bristles become hard...a few seconds back in the glue will dissolve the dried adhesive and you are good to go again.
A pot of glue - the type of glue you use is extremely important...it must be Liquid Polystyrene Cement
(I happen to be using Humbrol). This type of cement is very fluid - almost the consistency of water. It aggressively dissolves the top layer of polystyrene plastics creating a 'weld' effect between components when dry.
Please, please please, DO NOT
try using these types of cement, they are quite viscous and totally unsuitable for this process...you will just end up with a gloopy brush!
Finally you will require a bunch of plastic shavings, so save some of your empty sprues.
1.) First you will need to make the shavings; take a bit of plastic sprue and using the edge of your modeling knife scrape away from yourself repeatedly till you have a pile.
2.) Now, dip the tip of your paintbrush in the glue (just enough to wet it) then use the brush to pick up a shaving.
3.) Next, carefully dip the brush and the plastic shaving in the glue just a little...you will see the shaving immediately curl up and form a little blob on the tip of the brush.
4.) Apply the plastic 'goo' to the gap in your model. You will need to repeat this several times until it is filled. Remember, this technique is for small gaps and holes only
, if you have something more substantial I would use regular putty to fill the gap. I have used small chunks of plastic sprue in the past for filling larger gaps in plastic models but it takes much longer for the plastic to melt and you really have to do it in-situ on the model rather than using the dipping method.
5.) The biggest 'plus' for this method is that the the model is very quickly ready for filing/sanding as the glue takes moments to evaporate and the plastic hardens in about 5 minutes, compared to the hours you would need to wait for putty to cure.
6.) I always give a quick glue 'wash to the area once it's been sanded or filed, to smooth the finish.