we need clearance to install stem valves because the faucet hole was drilled too close to the sink. We need about 1/2" extra clearance. The counter top guys drilled too close to the sink and did not leave clearance for the stem valves. I'm afraid if I sand the enamel sink, underside that it might crack the sink.
If you could sand on the bottom of the sink - by the time you removed 1/2 inch you would be way through the sink thickness. So I think that the sanding of the sink is a non starter.
Since it was the counter top guys that screwed this up you should demand that they be the ones to take the steps to correct this problem. If that means ripping out the counter top to install a new one then that is a cost that they will have to bear.