There is none (that has been tested and approved by Van's anyway).
But this is experimental aviation after all, so you may be able to find something that will work just as well (by that I mean tank sealant is not necessarily the only thing that will work, but 16 years of service data has proven it to be a good choice).
Just some of the things that need to be considered (after all, this is a structural part of the elevators and trim tab...)
- The adhesive has to have good shear and peal strength
- It maintains that strength through a wide temp range.
- It remains somewhat flexible to allow for different rates of expansion/contraction between the foam and the alum. skin.
- It doesn't chemically effect the foam material
- ??????