Anyone have any good results from a way to clean out old gas tanks.

I have a metal tank on the surtrek and a plastic tank on the attex.

Also when I put a magnet in the surtrek tank there was a TON of like metalic mud like sludge. It would stick to the magnet and come off that with my fingers like mud. What could this be? (I went in with the magenet 25+ times and everytime came out full.)


Also does anyone recommend any sort of tank sealer/ coating once I get the tank clean.