I think GREENSTUFF would be the best for larger gaps, but for smaller ones, or imperfections in the model, I\'d use a product called GREEN SQUADRON PUTTY. Its a bit like toothpaste. You can make a sort of thick wash also to fill in pin holes or little bumpy areas.
http://www.hobbylinc.com/htm/squ/squ9055.htm
Should be easily available.