I have something based on a cylinder:
First, this is for 3D printing, so I’m not so worried about creating an n-gon here, since that is actually easier to deal with in printing. (I know they’re bad in rendering!)
I have the vertices of two loops selected. I’d like to create a face that connects the 2 loops without filling in the center of the cylinder. Is there a shortcut to make a circular face between these two loops without it including blocking off one end of the cylinder?
It looks like you should be able to bridge it. If you have loop tools turned on the shortcut is W: loop tools: Bridge
I tried right clicking for a context menu and “Loop Tools” is not on the menu. Are you doing that to join 2 different objects? I’ve seen that in a lot of tutorials and posts, but I haven’t seen how to join these two that are on the same object without just making one face that closes off the whole end.
Nevermind - did not see the “if you have loop tools turned on.” I had to install it, then it worked fine.
Hmmm… simple bridge edges should do… ( switch to edge mode 2 control E Bridge Edge Loops…)
You can sometimes get odd results if the faces of the respective edge loops are not pointing in the expected direction, I’ve had odd bridge loop situations in the past because of that.
Oh yes… didn’t thought of this… so then sometimes one bridging edge and dozen of hitting F1 ( assuming the F2 addon is enabeld )… or using the Grid Fill ( 
with apropriate done edges… )…
…sometimes… …but…
…not everytime…
So, basically, check the normals on the faces and make sure they’re all facing the outside.
At least, in a logical way, so for a pair of edge loops where there’s an “inside” and “outside” and one is bridging the top, make sure the inner polys face inwards, not towards the outside. Also, if you are getting odd behaviours with bevels, normals are a culprit quite often.
