Drip rail sealer

Been down this road. Do yourself a favor and use a two part sealer like Lord Fusor #129. I used it on my drip rails and again this past summer when I modified a steel hood for the blower. Had to re-bond the inner structure to the skin. Also used it to dress seams around the new opening. In that case my friend, who's an autobody supplier, brought several products by to try and agreed that the 129 would work best.

The 3M Fast and Firm (single part in a caulk tube) is pretty old school, takes forever to get hard, and if it's thick it will crack like Adam mentioned. The two part products will set within 30 min., are sandable, and just work really well.