black stuff behind sealant what is it?

[xpress]

i saw this sort of shiny looking black stuff inside the seam, the big question is, does anyone know what it is? is it welds, or some kind of non setting waterproof mastic between panels from the factory? thanks. now for the waffley bit ..

i have been de-rashing the o/s lower middle seam, bosch drill and thin wire brush has been the weapon of choice. the lower part of the metal as it meets the sealant seam had black deposit there that took a good while to get off and the brown rust dust flew out until i cleaned it out. it took a lot of the old sealant with it, i plan to stickerflex new sealant in its place after a good soaking with jenolite in the seams and that should keep things at bay for a while. i used jenolite and kept at it.

ps is it acceptable to primer the seam, then key it and apply the mastic afterwards, or mastic straight to bare metal? (the mastic seems to stick to anything).

[boatbuilder]

I think ideally I would use lechler epoxy primer and then sikaflex or tigerseal.

But there are a lot of different ways of doing it depending what products you have.