Hi guys, this is a noob question, but I'm hoping someone here knows the answer and is willing to share.

I downloaded an STL file of something with sides and a roof, but no floor (think of a plastic hotel piece from a basic Monopoly set). So when I printed it, it was somewhat fragile due to lacking any interior reinforcement. I thought it would be pretty straightforward to import the STL into Blender and then add a mesh plane to the bottom of the model, which would then allow my infill settings to provide internal reinforcement. But when I exported the combination to a new STL and printed it, nothing changed. So my first question is: Should this have worked? If so, I should try to figure out what went wrong. If not, do you have suggestions for the best way to accomplish my goal?

Thanks!