I have done a couple of attempts with a mold i created with the uni-mold. The mold was not post cured. When i make parts that is not post cured there is no problem, but when I create parts that i post cure i get a carbon fiber pattern in the mold surface. This pattern I have to sand down with 1200 paper to get it flat after I demold the part.

You should always post cure above the temperature that you cure at, sounds like the gel coat is under cured if the twill is leaving marks in the mould it must be still soft. I'm not sure what type of system you used to make the mould, I would follow the data tech sheet for correct curing of the mould.

If it is an exotherm type system it may well be that it did not get to the correct temperature or it was slightly under catalysed a simple barcol hardness test would help.

Heat cure should fix the problem. Good luck.