Best product for sanding down plaster?

Hi everyone, I've plastered over a gap which is about 2" wide & 15" long, it's dry now & i've got lots of little dimples in it & that'll obviously show up badly when painted. I picked up some sand paper which has made everywhere smooth but it doesn't seem to be taking off any layers... show more Hi everyone,

I've plastered over a gap which is about 2" wide & 15" long, it's dry now & i've got lots of little dimples in it & that'll obviously show up badly when painted. I picked up some sand paper which has made everywhere smooth but it doesn't seem to be taking off any layers to get rid of the tiny dimples/holes.

Am i using the wrong type of paper? Not sure of the name but it was the yellow type, should i be using the black wet & dry stuff?

Thanks in advance. =)
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