myles - @printedsolid,
Many thanks for the advice, I will give it a try, will let you know how I get on

ralphferro - SketchUp has a function where you can push one shape into another.
For example, you can push a sphere into a cude, pull the sphere back and and the sphere will leave and indentation of itself in the cube.

myles - @ralphferro, many thanks, unfortunately I don't seem to be able to load or import an stl in sketchup.
@printedsolid, I have also tried Blender, but no luck.
A customer sent me a stl file of a scan of a face, so, floating above the table there is a paper thin mask of the face, face up. I can't find a way to make it solid and therefore supportable when printing.
Originally the brief was to print a mould of the face ( negative ), but the brief changed to printing the face (positive ). The job was needed in a hurry, and I am now probably too late, but I would still like to work out how to do it. Perhaps I should ask for it in a different format.
Any more ideas much appreciated

jesse - OpenSCAD might work

myles - @jesse, thank you, but can I import a stl into Openscad? I tried dragging it into Openscad , but it crashed.
I am a little suspicious of the stl file I was sent, it also looks a bit odd in makexyz's previewer. Slicer doesn't like it either.
Would it be too much to ask if someone could take a look at it, tell me if it looks ok? Customer not replying anymore, so it is just my aim to puzzle it out for future reference.
Much appreciate any help

myles - SORTED!
Have received a new file, prints first time.
Thanks for the help folks

peter19 - I am here also needed the awesome article and now i am invite here http://freeimvucreditshacker.com/ all the online user and getting more online games.