I’m trying to print this part in a specific orientation as per the creator’s recommendation. However, when I slice it, this is what I see:
Try turning on “Detect Thin Wall”.
I tried it already, no change 
Did you actually “Try” to print this and it failed or are you simply taking the slicers word for it? I ask because what you see is not always what you get. If you don’t want to commit to wasting a lot of filament, then try using the cut tool and cut out a section of the model and re-slice to verify that it slices the same way. If not, go back and re-cut until you get a smaller section that will slice the same way as the full model. Then simply print the model to prove or disprove that the model isn’t forming correctly.
Now having said that. Can you share the source of the model? I ask because if it’s your design, you will want to increase the wall thickness in CAD. However, one often finds models on the various sites that have never seen the light of day, they are only CAD constructs with photo realistic renderings and are without proof of actually been printed.
Here’s an example of a fictitious model that the designer most likely rendered using either Fusion360s render tool which can produce a very photo realistic image or a combination of photoshop. But what you won’t see on his profile is an actual print. This is clearly a fraud. What’s sad is that this sort of chicanery requires more work than simply printing it. But if you don’t own a printer and perhaps you are using the one at University, then how better to show off your CAD resume than to post an image like this. I’m not aware of any filament that has a chrome finish. 
Spray-paint perhaps? Or Digital forgery?
https://www.printables.com/model/180557-low-poly-wall-shark
It’s not my design, and many others have printed it up.
I’m not at home yet, but I’ll try your cutting idea when I get the opportunity.
For the hell of it 
Try changing the model orientation.
Exactly what part do you think is missing?
If you are referring to the surface shown in red, that is not missing, it is simply shown in a different color because it is being seen as a top surface. It will still print.
Tip the model toward the back and that surface becomes outer wall orange, but another surface may become more horizontal and turn red.
I have it specifically oriented that way because that is what the person who designed it recommends. The shark teeth are very thin, and can get screwy if they’re printed in a different orientation.
It’s not that it’s missing. I know that it’ll print it. But that section is “sparse infill,” which shouldn’t be on the exterior.
I’ve already tried rotating it in the slicer, and when I do so the problem goes away. However, I would like to print it in the original orientation because that is what was recommended by the designer.
I feel like no one’s addressing my original question which is: is there any way to manually change an exterior face to a wall?
No, it is “Top surface”. Toggle the Display check marks.
Changing Sparse Infill changes nothing.
Turn off Top surface and you see the underlying Internal Solid Infill.
No, THIS is the original question:
There is no “exposed section”.
An exterior face may be a wall or a top surface, the difference is only the orientation, which you are unwilling to change. The visible portion of a wall is mostly the side of a layer, the visible portion of a top surface is mostly the top of a layer.
Since you apparently have not attempted an actual print using the recommended orientation, you are trying to correct a “problem” that is only in your mind.
Well damn, you’re absolutely right. I didn’t even notice how similar the top surface color is.
Alright, so it turns out this was a big nothing burger and I wasted everyone’s time. My bad guys 
