The sealer remains flexible forever-awesome stuff, I've been using it since it was first introduced in 89'. Make sure to use the urethane version that comes in the breakfast sausage type tubes-they are called chubs, there is also a solvent based ultrapro that is junk-stay away from any solvent based sealers. Prime your parts with epoxy, apply a bead of the sealer on all lap areas and bolt the box together, wipe off the excess the squishes out-this will provide the best corrosion resistance. Most people assemble then apply the sealer to cover the edge of the seam but the seam will still have an open void this way and if there's any entry point for water you could eventually end up with a hotspot for corrosion. You've got a perfect opportunity to assemble that box right.