How are wing tanks sealed around the internal wing structure? I can easily grasp the idea of tanks of fuel inside the fuselage of an aircraft, and the ease of sealing such tanks in a relatively standard shape.  However, I'm having a hard time understanding how wing tanks could be fit around the internal structure of a wing, as I envision the inside of a wing (especially nearer to the fuselage) to be a honeycomb of structural pieces that would be very difficult to seal around or find any large volumes of contiguous space for fitting tanks.  Is the wing structure itself the fuel tank, or are there separate fuel tank bladders that are somehow fit into the wing voids? 

For my question, I am not concerned about military fuel tanks which might have special materials in the walls which could seal off a leak due to combat damage, but rather just non-military aircraft.
The tanks are generally sealed with some kind of putty/resin/epoxy like material. Keep in mind that only the joints between exterior panels need to be sealed. On many aircraft this is done through inspection panels. The material is, in a manner of speaking painted/applied.  

Here is an example from a Mooney, which are pretty infamous for having in wing tanks


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Tanks on larger aircraft are not all that different, much like this one off a 747


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its worth noting that tanks don't need to be continuously open, they can have structures with in them as long as the fuel is free to flow around. Large and small tanks alike may very well have structural elements inside with flow holes drilled to allow fuel to move around. These elements also help reduce the effects of large quantities of fuel sloshing around as the aircraft changes attitude.