If the surfaces defined a watertight volume you could use surfsculpt to create a solid from the surfaces. You model is not watertight. Note the gap in the image below. You are correct that you should have used solids and not surfaces to start with. You should also make use of layers in the creation of the model. For example, create the wire frame model on one layer and the surface or solid model on another.
I suggest deleting all the surfaces leaving just the lines in 3D. Define a new layer with a different color and create boxes (solids) and extrusions to define the geometry. Remember that you can use union and subtract to simplify the process. You might want to set the visualstyle to X-Ray
YOu could start with an extrusion like I show below then subtract the blue boxes. Create a box or extrusion to subtract from the inside if the object is hollow.
Good luck!
As can be seen here your model does not define a watertight volume as there are hole at the top and bottom of the gear teeth.
To fix this your need to define additional surfaces that will make it watertight when using the surfsculpt command.
The first step to do is to explode the arrayed teeth converting them to surfaces.
Next create planes to define the top and bottom of the teeth. Since you want to maintain the hole through the gear you need to have a hole in these planes. You can make the hole by using a Boolean subtract of a cylinder that has a radius less than the gear OD and greater than the hole's minimum ID.
After the Boolean subtract you are left with this.
You can now use surfsculpt to make a solid of the geometry.
