Yes I think you can put the rust over the whitewash especially if the tank's been semi-submerged for some time. The rust will start where the paint has been shipped away, but the effects of time and weather will mean that the rusted areas will tend to spread - what happens is that moisture finds it's way between the paint and the metal underneath and starts to rust the steel under the paint. If the moisture in this space freezes than it will cause the paint to flake off even more when ice forms under the paint. Bright yellow orange rust may show in areas but I think the darker brown colours will tend to predominate in this case
Concerning the second question it will depend on the time elapsed and to some extent the weather, but after just a few days I would still expect to see some areas of slighly dulled bare metal showing.
