Stuart,

Look at the other thread in this forum about sealing a roof "gutter". A two-part sealer is mentioned - I now recall that we used on the interior seams a two-part sealer that cured to a paintable semi-flexible(?) state (my friend worked in a auto-paint shop). Sorry, but I do not know the brand of product that was used (it came with its own two-part applicator "syringe").

Tim