While editing an official car's 3d model, I created some holes in it. I patched them up, creating rather rough and ugly shape. Is there some modifier to make them smoother? Or I just patched that hole in a wrong way?
That black triangle looks like the normals are facing the wrong way, or you simply have a hole there. Try selecting the facw and pressing CONTROL-N (cant remember if this is the right key or not, I just go by muscle memory) and see if it looks better.
If you look at the sidebar on the left you should see an option for either smooth or flat shading, click on smooth and see if you get the result you want.
on the face the normals are facing backwards. do this: go to edit mode> A (select all) >Shading/UVs>Re-calculate. if that doesnt work then select the face and hit flip direction.
