I like your bridges, and I too had a lot of fun painting those, I don't know why.
For light leaks overall I just sprayed a heavy coat of silver inside, and one outside.
Nothing gets through metal paint. Then a primer outside for the real paint, then paint.
No leaks except at some joints of course and those are small. Filler/putty, silver,primer & paint
each.
Keep taking pics and posting. I'm enjoying watching your build.