Rust in tank which treatment?

Hi Guys

Have any of you had to treat your tank due to rust. I have had to leave bike standing for a while, pulled it in to get it ready for use to find the seal on the filler cap had gone hard, and the drain from around the filler neck was blocked! I have had a look in the tank with an endoscope and it is not too bad. Have cleared the drain, so need to clean the tank out, it is just surface rust, not flaking, so I do not think I need to use a sealer.

Any advice would be great.

Cheers

Steve