never had this happen to a mold ,
but for other items of aluminum - automotive or what ever .. you can sand um flat .
It will take off a very small amount unless it is warped bad ..
you need a perfectly flat surface ,, Glass table top ,, some counter tops .. pc of steel that has been milled flat ..
use wet dry sand paper ,, not sure what grit .. maybe 1000 ???? not course at all ..
tape sand paper down grit side up and move mold half in a figure 8 motion with your hand,,
you should see the high spots on the mold getting sanded rite a way .
would be a good Idea to get more opinions on grit of sand paper.
for clear coat Car paint repair I have used as mild as 2000 grit