I'm pretty sure water gets in no matter what you do. It does seem a little counter-intuitive. But there should be a hole in the bb shell to let it out. And also, I had a friend that got water in his downtube and it rusted out. It was a fillet brazed frame. I would like to know if it had any holes into the bb shell, but I'm wondering if it would still trap water if they hole was centered in the tube.