Have just had a bit of plastering done in a small boiler cupboard and it's not very neat in some places, as the space was absolutely tiny (about 2m2 with a wall-mounted boiler in there as well!). I'm just wondering if I can simply sand down the places where I can see trowel marks? If so, can I use my electric sander, or am I best off doing it by hand? What grade sandpaper do you recommend? Thanks.