Originally Posted by Nbigger
I noticed behind the chair rail there is some areas that I can see light. They don't seem to be rusted through but just seams that did not get sealed.
If it's like mine, those are spots where the floor (and the part of the chair rail that bends over the edge of the floor) has rusted all the way through. But perhaps your outer wall has just separated a bit from the outer flange of the chair rail.
I'm not super-worried about these spaces behind the chair rail. I think that as long as you stop the internal leaks from the windows and roof, no more water will get down into these cavities and they'll be fine (I'm also going to patch and seal my holes from below to prevent anything from splashing up from beneath). I've considered maybe filling them with spray foam, with a single 200-board foot kit, since I have some other cavities I want to fill with this - not for insulation purposes but to ensure water can't get into the cavities.
If I needed to seal between the outer wall and the chair rail (like what may be happening on your bus), I would try using a long-handled hobby paintbrush to paint on Dynatron-550, using a mirror to help me see down in there.