The F-111, unlike many military and civilian aircraft, does not have bladders to contain fuel.

Various spaces within the F-111 fuselage (image 1) are utilised as integral fuel tanks, however, to ensure their integrity, they have to be sealed. When the F-111 was designed, a sealant that could withstand environmental conditions arising from supersonic flight was
developed to bond fuselage panels.

Fuel leaks were soon discovered after the F-111 was introduced into service. Those leaks were traced to the fuel tanks where it was found that the panel sealant was extruding and breaking the seal on the inside of the tanks. Initially these cracks were patched on a piecemeal basis until it was decided to completely strip the internal tank sealant and reapply a new seal. The bonding on the fuselage panels could not be rectified without dismantling the whole aircraft.

Basically this means the F-111 was one big fuel tank and the only thing stopping the fuel leaking out was a special sealant applied along the places where the aircraft structure met other parts of the structure and anywhere else the fuel could potentially get out.

The sealant used was a fairly toxic 2 part compound much tougher than your average silicon type sealants you might purchase at your local hardware store. Some believe the chemicals in the sealant were just as toxic as the other chemicals used in the program and because of the ineffectiveness of the personal protective equipment used it was normal procedure to have various parts of ones body covered with this sealant.