OK, I now have a part I am happy with - how do I get it watertight for 3D printing?

It's composed of:

- two solids from the STEP part I imported
- the eight networks

I've tried:

- joining the networks together
- separating the solids into surfaces and then joining those surfaces with the networks (seems doing that converts the surfaces back into the two solids)
- separating the solids into surfaces, selecting only all of the surfaces and then exporting

Any ideas what the steps are? Thanks, Andy