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I wouldn't use sand paper at all, it will hurt the pads on your hams feet before it does anything useful for nails.
Which species of hamster do you have?
Syrian hams nails don't normally grow that fast, a sand bath will normally help keep them down & some natural rough surfaces in the cage like wood, or something like a tile turned upside down, natural stone like granite tiles, smooth stones won't do anything so not really worth bothering with.
If the nails have grown really long then they will need to be trimmed, best done at the vets if you've never done it yourself before, as things to help in the cage are only good for maintenance not really for trimming long nails.
Dwarf hams nails (especially Russians) do grow very fast & usually need regular trimming.