Turning joined surfaces into a solid.

 From:  Gord (NEOMEGA)
9865.1 
back on my ship modelling again! I have made a hull but it has become joined surfaces rather than a solid. I am at a loss how to fill it in so I can chop out parts.

I have tried booleaning a solid on, but it just turns the solid into a joined surfaces rather than the other way around!

Thanks!