If you want to protect the inside of the tank after draining it, consider:

-Food-grade mineral oil (used sparingly, only if storing dry)

-Silicone spray (non-toxic, water-repellent, and rust-resistant)

-Rust-inhibitor spray or metal sealant specifically designed for galvanised steel (especially if storing long-term)

Always make sure the product is non-toxic and pool-safe if you plan to refill with water later.