I was curious - perhaps if they went back to painting the external tank. I don't know what the paint consisted of nor if there would be much to be gained from the tradeoff of weight nor how much additional strength it would provide.
I don't know enough about how the painting worked, but I know that by not painting them, they saved a ton of weight. But, perhaps they could seal these critical places with a sort of external coating, similar to the rubber casings used on cruise missiles. It seems most of the foam flakes off around critical junctures, and having a lightweight but druable external coating that can withstand the pressures involved might help solve this problem.
I'm sure that NASA has already thought of this, though. At least, I certainly hope they have...