I would recommend Polyfilla:
This is the special formula for deep holes and cracks from Polyfila. It's really easy to do:
1. Pour the Polyfila on the base.
2. Shape it the best you can with a spatula or any other instrument.
3. Let dry on surface (a few hours) The idea is to simulate a crust on the surface with smooth interior.
4. Use the models and figures to make the imprints and footprints.
5. Let it dry overnight.
6. Use sand paper and or dremel to sculpt if desired!
Pretty convincing results and it is still white after several months. The special formula for deep holes will not crack over time.
Hope this help.
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