Originally Posted by Pigeoneer
directly on the overlap before riveting.
This is the best way to seal two overlapping sheets because it mechanically squeezes the sealant between the two layers, closing the seam even if the adhesion of the sealant fails (it essentially creates a gasket). If you seam-seal after the riveting, you're relying entirely on the adhesive properties of the sealant to provide watertightness (and this may be adequate, it's just not the best possible).
That being said, I imagine that doing this for large sheets like what are used in a roof raise would be difficult to manage. I've only used the technique for riveting sheets that are small enough for me to apply the bead of sealant, place the piece with adequate clecos, and wet-rivet everything before the sealant hardens (like, a 24" x 24" patch over one of my exit hatch openings, for example). Even with pieces this small, I have to keep the sealant (I use Dynatron-550 as do many others) misted with mineral spirits to prevent it from skinning over too quickly.