How to get sharp edges with subsurf and concave Ngons Yes, I know, this is frequently asked. Edge loops and creases solve most issues with this modifier. My problem, though, I can't seem to find anything on. I want the corners within this open rectangle to be sharp. According to blender's online manual (v2.79 is what I'm looking at), it's something to do with concave Ngons (which I know nothing about). Also, this is for 3D printing (hence the solidify modifier) if that opens or closes any options.



I know that my topology isn't the most even, I intend to fix that right after posting this. I have tried adding vertices and moving them towards the corners, as I have done in this section:



But that doesn't give me desirable results (plus messy topology to get it to look even halfway decent), and it's even worse in the area I'm trying to fix currently. 

Any ideas? I apologize if there is a similar question that I'm just failing to find. Thanks in advance
I also struggle with this. I have two tricks that might help:

You can make the corner of a hole sharp by creasing an additional one of its connected edges:



But if your surface is curved, this might create an ugly sharp discontinuity.

So instead you might try the crease options on the solidify modifier:


Again, it can get weird when the surface is curved, but I find it helpful in most situations. If you don't want your surface to be made solid, you can set the modifier to only generate a rim, and make the rim vanishingly small.

Hope this is helpful. If you find anything useful, please share it!