I have a long way to go learning FreeCad.... and unfortunately limited time to play with it. For water tight issues on the stl files, I use NetFabb ( free online tool). 9 times out of 10 it fixes the stl. As to why it happens.... seventypurple? I've exported the same design to stl twice and had watertight issues on one but not the other. Don't know why though
It's comforting to know that I'm following the same steps that those who knows.
I've been talking to people that do have a 3D printer and several, but not the most have a 3D printer only to print things taken from Thingiverse without any design or modeling idea nor experience.
In these few days I've been trying different on-line services to repair STL files and it seems that Netfabb is the simplest and quickest.
Thanks a lot for your support.
[Edit] I've exported the same model from FreeCAD to stl with a faster/newer PC + Windows 10 and now the models are watertight for the slicer.