Steel bikes don't rust from being ridden in the rain. You mentioned above that water was draining out of the bottom after you rode. That is the water leaving through the holes that allow ventilation to dry the inside of the tubes. As long as water doesn't sit in the frame for weeks/months, rust is not a problem. For insurance, inside the tubes can be treated with frame-saver or some similar rust preventing coating. I rode my Bridgestone RB-1 in the rain many times during the 18-1/2 years I rode it. It took a car hitting it to take it out, rust didn't get to it.