I had rust in my 1978 Puch Maxi tank although nothing as bad as what you have pictured here. I removed the petcock and plugged it with a cork. I then added Metal Rescue to the tank and let it sit for 48 hours. Metal Rescue is a water based product and will not hurt the tank. I then drained the fluid and repeatedly flushed the tank with a garden hose. I was shocked at what came out.