There is a series of access panels under the wing pretty much the length of the span,the size and shape of an oval dinner plate. It would suffice to say that the corpulent would have a hard time entering. Getting in is only half the fun,as you must remove internal baffles and navigate the labrynth like interior. A 757 size A/C and up is not too bad,but anything smaller is quite a test of litheness,avoiding damaging wiring,pipes,probes as you slither through...it's no job for the claustrophobic. The sealant used is a type of PRC,chemically cured,that is specific to the job it's done. (we don't use PRC for windshield aerodynamic sealing in the tank). It's a two part mix. Center tanks,where applicable,are generally more "roomy",but are still a PIA to enter. Entrance is generally through an airconditioning pack bay (after removing copious ducting),or through a removeble wall from the wing tanks. On some A/C,the center tank actually resides in the inboard sections of the wing as well as the fuselage. I hope this answers your questions. Later!