When filling voids with epoxy, leaks are commonly caused by either the seal failing (tape/caulk) or the epoxy finding its way through an unsealed crack.
In the first case we have found that applying the tape to the bottom of the void, then ensuring the seam along the inside of the tape is sealed will greatly reduce the chance of a failure. The best way to seal the seam is by applying a thin coat of epoxy along the inside of the void and tape seam. Allow the epoxy seal coat to gel before filling the void.
The second case can be a bit more tricky as cracks in wood can be quite intricate and complex. In most scenarios, a seal coat of epoxy will be enough to seal fine cracks that could allow the epoxy to leak. Larger cracks can be more difficult to seal due to the low viscosity / slow curing nature of the casting epoxy. In these cases, a thicker / faster-reacting epoxy like our Quick Cure 5 or 15 can be used to seal potential leaks.