Removing Heavy Rust from a Cast Iron Drain Insert

I have a Cast Iron Drain Insert in my laundry room floor drain that has heavy rust on it.

It is removable and it is a heavy cast part that is still very solid except it has a lot of rust and dirt on it.

I would like to clean off the rust then paint or coat it so it looks nice.

Any suggestions on how best to remove the rust and what to use to paint or coat it would be much appreciated. I have read about using Phosphoric acid, but I'm not sure if that is the best method nor do I know what % of acid to use.

There is probably NOTHING that will "remove" rust from cast iron, except a chisel or wire brush wheel. It will probably never "look nice", but it may look better, even though it will be pitted. Paint it with any spray paint.