In the past I drilled a hole in my BB shell to let water drain out. I chose the lowest point of the BB shell, put two small (1/16" 1/32"?) holes in. I haven't done this recently.

You can also do some checking for brazing holes in the chainstays(where the tube has a pinhole to help vent brazing gasses/pressure vent) and hang the bike such that those holes are the low point.

For rust proofing use the sticky brown spray sold by Weigle as well as any automotive store (sold as "self-sealing" and not recommended for painting over). Spray liberally inside the tubes.

cdr