If you can get it over your way try Humbrol model filler
http://www.humbrol.com/model-filler-31ml-tube.htmlYou may find a version by Squadron green putty - they also do a white / grey version but I've not tried this.
A couple of points to note:
a) These stink of solvent.
b) They can melt models if the carrier separates into a thin clear liquid - use only the gritty parts and bin the tube ASAP
c) They can stick to anything - carpets / shirts / mouse mats - yup done the lot before now
I have never managed to completely use a tube yet. I find they go hard or separate and then I buy another one as once it gets thick its next to impossible to use.
Sands and paints very well - a lot simpler to use compared to green stuff as it bonds better and is more fluid to mould / ram into cracks.
Works on metal as well by the way but can pop off if the model gap has smooth sides.