Knit your surfaces and use the the make solid option.
Hi Alan, can you upload your part for us to look at? I think i know what you want but I want to make sure. It is easier to look at the part in my opinion. Tom
If you can offset your surfaces and the resulting surfaces are water-tight, then you can just use the Thicken command to make the interior solid. Just uncheck the box that merges the bodies.