Otherwise, on a warm day..
Pull the tank out of the car and drain the fuel into a bucket..or somewhere else safe. Use some pantyhose to filter it as you transfer it. Fishnet stockings aren't appropriate for filtering.
Pull out the sender unit, and blast out the tank with soapy water and a garden hose..then flush with metho..Let it dry.

Then pour in some rust treatment stuff..It probably only need to coat the bottom. scrub around inside where you can, it makes the rust treatment work better.

Let it dry as per rust treatment instructions.

Rinse out with metho to get any strange residue out. Then put the tank back in the car and fill up the tank (To minimise petrol contamination concentration).

That's how I fixed mine.

I used an Australian rust kill product called "Emertan" Which is a Tanin Phosphate solution.

Chris