What I would like to do is just cut and paint new plywood (on the ground) and just nail/screw it to the old soffit plywood. I would like to do this for 2 reasons: I wouldn't have to strip the lead paint and the 2 pieces of plywood would provide more noise insulation.
What worries me is that moisture might get trapped between the layers and rot out the wood. I am not against priming the old and new wood if that will help protect it from moisture. Will this work? Will priming be enough or do I have to design it so air can move between the layers?
I am willing to go one step further if it will help and is safe; I've worked with a sound barrier called Homasote 404. What I could do is cut the 404 and install it between the 2 layers of plywood, would that work? Or would it be better to cut strips of the 404 and stagger it so air can move between the plywood layers?