Dikran, Are you talking about JewelCad or JCad?
JCad is the design software that comes with the Meiko SLA machine.
It doesn't do STL files well at all. The Meiko machine uses SLC files
from the JCad and doesn't seem to be particular about whether it is
printing a solid or not.
Your Viper should be able to take "watertight stl" files with no
problem. The Perfactory will also.
I am not sure if JewelCad will produce watertight models. Most of
the folks I know that use it use CNC milling machines and don't need