Ok the best way to do this is.
Empty and remove your tank you will need to contact Frost Restorers.
You can get a kit of them that will make your tank like new.
The kit costs £44.00.
Or you can just get the tank sealer on its own.
Empty and remove the tank then put some pebbles in the tank with a bit off fuel and shake it about, this will clean it out,empty the tank then pour in the sealent.
Sealent on its own will cost you no more than £20.50.
Have a look online at
www.frost.co.uk
WARNING NOT A GOOD IDEA TO SMOKE WHEN DOING THIS JOB OR YOU MIGHT NEED MORE THAN JUST A NEW TANK.