[nevardmedia]

Another option to plaster (which I tend to find sticks to everything you don't want it to) is Humbrol Air Clay. It's ready mixed, a bit like putty. You simply push it into place (a stiff brush is ideal). Great for working around track and sleepers. Dries in about 6 - 12 hours depending on how thick.

I used it here:
http://nevardmedia5.fotopic.net/p37775945.html

Downside is that it can crack a little when drying. No fear, just smear some more clay into any cracks and blend in with a wet finger.