I'm going to assume that your metal water tank needs the loose paint and rust removed. To do this I would use an electric grinder with a sanding wheel, something rough. This will remove anything that is loose. Rusty areas can be treated with a rust converter, available at auto paint supply stores. After all of this you will need to prime the tank before painting. A good universal all purpose oil base primer should work.
The primer and paint could be applied with a roller or spray equipment depending on the tank size and what equipment is available to you. If you use a paint roller, roll from side to side instead of up and down.