Going from AC3D to .stl

[MikeM]

I'm a long time, but occasional user, of AC3D - mostly for rapid sketches of industrial kit and machinery - so no 'pretties'. I now want to try and make some pieces for a laboratory experimental rig using 3d printing, and rather than struggle to learn Solidworks, would love to do it in AC3D. However there seem to me to be a couple of issues I don't understand.

1 How do I make the dimensions 'real world' instead of grid units? If I want a 100mm disk - I can make it 100 units diameter, but how does the conversion to .stl know what this means?

2 What happens to surfaces that are hidden when you merge two pieces. Even if you weld the vertices there are internal surfaces, often hundreds if trying to make smooth curves to accurate dimensions

Thanks in advance for any comments