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The problem with sand paper is that for them to be on it long enough to trim nails at all it would probably hurt the feet so it's not really a good idea.
Once nails have grown too long there really isn't anything for it but to trim them, by the vet if need be.
For prevention rough natural surfaces in the cage can help, things like terracotta pots, granite or other stone with a slightly rough surface, plenty of wooden toys can help a bit too.