Ive done that before.
Dry the tank right out. Hot air gun or leave it in on the hot water service or next to the heater.
A good hand full of blue metal, hand picked, gnarly shaped ones with pointy bits. Was them up and dry them out.
Then use Kerro or diesel with the stones. That way you can shake rattle and roll for weeks.
Dont use water as thats what made it rust in the first place.
