IMHO silicone sealants are worse than useless. They work just long enough for you to think that they are doing their job.... then leak.... ruining your hard work.... and leaving a surface that cannot be cleaned properly for anything else to adhere to.

Use epoxy thickened with micro-balloons. It will not fail except under bizarre circumstances. If you really screw up and have to move the bulkhead, then get yourself one of those sonicrafter tools. They work great for cutting out a bulkhead, and a bunch of other uses (blush).